Image source, Getty ImagesThe club says recruitment is already under way to replace Neil Harris.
But who would you like to come in?
Come back to this page on Wednesday to find a selection of your replies.
Millwall manager Neil Harris will leave the club after their Championship match at Middlesbrough on Saturday.
The news comes with the Lions 11th in the table, six points short of the play-off places.
Millwall fans have been reacting to the news and you can share your thoughts as well.
Here are just some of the messages from supporters:
The Lone Gunman - Good or bad he is a MIllwall Legend. Much respect. Good luck Bomber.
David 'The Clash' - I am just so disappointed. It really doesn't make any sense. Clearly something has gone on behind the scenes. Who on earth can the club get in now? I just sense something isn't right here.
Peter - Could never doubt his commitment to the club but IMO he doesn't have what we need to continue to develop a side with a real chance of promotion. Good luck wherever Neil.
Wambambigalo - Nothing to see here. He's always said he only came back because the club needs him. He's leaving because he's had a bit of stick from the fans. In the future if the club needs him he'll come back again, he's Millwall through and through.
Oogbwqsy - Thats a terrible decision, Millwall have been great this season.
Image source, PRESSBOX GRAPHICSMillwall's 1-0 home defeat by Coventry on Saturday ended a nine-game unbeaten run.
The Lions have only lost back-to-back matches in the league once this season.
Sheffield United have the third best away record in the Championship but have only won two of their past six games on the road.
Millwall have won three of their past four league games against Sheffield United (L1), with each of those three victories coming via a different scoreline (1-0, 2-1 & 3-2).
Sheffield United have lost five of their past six away league games against Millwall (W1), winning their only match during that time 3-2 in September 2018.
Millwall have won each of their past three home league games played in midweek, all 1-0 (Tuesday, Wednesday, Thursday); the Lions haven't won four such matches in succession since October 2001 (5).
Sheffield United have won just one of their past 18 league games against sides from London (D5 L12), conceding more than two goals in each of their past nine such fixtures.
Millwall defender Jake Cooper has scored five goals across nine prior league meetings with Sheffield United, more goals than against any other side during his career.
Image source, Rex FeaturesBilly Mitchell has made more than 100 appearances for Millwall
Millwall's Billy Mitchell made his long-awaited return from injury in Saturday's 1-0 home defeat by Coventry.
The midfielder appeared as an 80th minute substitute having missed the entirety of the campaign so far because of a thigh injury sustained in pre-season.
The 23-year-old is set to be included in the squad for Wednesday's home game with Sheffield United but the Lions will be without Josh Coburn and Daniel Kelly until the new year.
On-loan forward Coburn has broken a metatarsal bone in his foot while Millwall manager Neil Harris has revealed midfielder Kelly damaged an ankle in an under-21 game at Watford last week.
Harris is hopeful defender Shaun Hutchinson could be available for Wednesday's match at The Den.
"He's tweaked a hamstring so it won't be long assuming his recovery goes at the pace it is, whether the Sheffield United game is too soon we'll have to see but we're hopeful [he'll be back] at some stage, whether that's Sheffield United or Middlesbrough," said Harris.
Millwall will definitely have defender Japhet Tanganga available on Wednesday after he served a one game suspension at the weekend having picked-up five yellow cards this season.
Image source, PA MediaNeil Harris' side remain 11th in the Championship after the 1-0 loss to Coventry City
Millwall manager Neil Harris says his side 'lacked a cutting edge' in their 1-0 home loss to Coventry City.
The Lions failed to register a shot on target as they suffered a first loss in 10 games.
"I thought first half we were the better team, but lacked a cutting edge and a couple of chances we did have we put over the bar or wide," Harris told BBC Radio London.
"Second half I thought Coventry were better than us until right at the end.
"I thought the last five minutes summed us up as a group really - we got into really good areas, just lacked the quality to hurt teams at this level.
"That's where we've always searched as a football club, and we've had it so many times this year where we've had the quality and the expected goals has been really high.
"Today we struggled, and at the end of the game a final moment - the ball comes into the box and I've got my centre forward and wide player standing outside the penalty area."
Image source, PressBox GraphicsUntil recently, Millwall had developed a consistent habit of winning matches 1-0 - but it looks as though 1-1 has now taken over as the Londoners' favourite scoreline in recent weeks.
The Lions have drawn three games in a row with a goal apiece, against Stoke, Sunderland and Oxford, within a nine-match undefeated run that has taken them to 10th in the table, five points shy of the play-offs.
Visitors Coventry have also got into a familiar pattern of late, with three of their last four games ending 2-2 - the most recent of those coming at home to Cardiff last weekend in Frank Lampard's first fixture as Sky Blues boss.
● Millwall have lost each of their last three league games against Coventry City and will be looking to avoid losing four in succession in the Football League for the first time.
● Coventry City have lost just one of their last six away league games against Millwall (W3 D2), scoring 13 goals in total during that time.
● Since a 1-0 defeat at Cardiff at the start of October, Millwall are unbeaten in their last nine league games (W4 D5); they last went 10+ games unbeaten in the Football League between January and April 2018 (17).
● Coventry have conceded two goals in each of their last five league games (D3 L2); the Sky Blues last endured a longer league run of conceding 2+ goals in November 2014 (7).
● Four of Millwall's last five Championship goals have come from set pieces (2x free kick, 1x corner, 1x throw). Excluding penalties only Derby County (11) have more set-piece goals in the Championship this season than Millwall's eight.
Image source, BBC Sport
Image source, Rex FeaturesJohn Docherty was a winger in his playing days, before managing Millwall, along with Brentford, Cambridge United and Bradford City
Millwall manager Neil Harris has paid tribute to former Lions boss John Docherty, who died this week at the age of 84.
Docherty is the only manager Millwall's history to take them to the top flight of English football, leading a side featuring Tony Cascarino and Teddy Sheringham to the old Division One in 1988.
"What a legend of the football club," Harris told BBC Radio London.
"What he achieved as a manager is something that we have all since tried to emulate.
"We want to be the next one to take Millwall to the promised land."
Docherty was in charge for nearly 200 games across two spells, from 1986 to 1990, and then again in 1997.
"I was really lucky to have met 'The Doc'. What a good guy," Harris added.
"He just oozed class and presence. It is so sad that he has gone, but his memory will live on for a long, long time."
The club will hold a minute's applause and black armbands will be worn in memory of Docherty in their Championship game against Coventry City on Saturday.
Image source, Rex FeaturesMillwall manager Neil Harris is demanding even more from his squad after extending their unbeaten run to nine matches.
The Lions conceded an 85th minute equaliser in the 1-1 draw at Oxford United on Saturday to record a third successive draw but remain in the top 10 and very much in the play-off conversation.
"The football club and I have to remind ourselves we are nine games unbeaten and how far we've come in a 10-month period," Harris told BBC Radio London.
"But the demand from me to the players is we don't want to settle for what we've done we want to be better all the time."
Millwall are five points short of the top six with back-to-back home games against Coventry City and Sheffield United next up.
"I have to look at the positives and say we played so well for 45 minutes and we got the blend right between being an aggressive Millwall team who can defend in shape but also we had some real excitement to our play and that's what we want to be," added Harris.
Image source, Getty ImagesIt is one of the most eagerly anticipated dates in the football calendar as Premier League and Championship clubs join the FA Cup action from round three.
The draw will take place on Monday, 2 December from 19:00 GMT and can be watched live on BBC Two and BBC iPlayer.
Image source, Rex FeaturesThe floodlights eventually came back on, but there were power issues in other areas of Fratton Park
Millwall manager Neil Harris says he can understand everyone's frustration after their Championship match Portsmouth was postponed because of a power failure at Fratton Park.
Millwall are on an eight game unbeaten run but Harris accepts it was an issue out of everyone's hands.
"2,000 Millwall fans have travelled down here and they were hugely respectful and it's frustrating for everyone," Harris told BBC London.
"I hope everyone can understand health and safety comes first but you can't control the weather and you can't control power grids so we just have to accept it and move on.
"I hope fans can use their tickets at the rearranged game but they don't get a refund on their travel so I feel for them and hope everyone gets home safely.
"Portsmouth were very good, as was the match official Dean Whitestone and we had as much information as possible until the security and police have to take the matter out of everyone's hands and make the decision that's in the best interests of the fans."
Millwall are next due to visit Oxford United in the Championship on Saturday, while Pompey travel to Swansea.
Image source, Rex FeaturesPortsmouth boss John Mousinho was hoping for a second successive home win before the floodlights went out at Fratton Park
Here's more from Portsmouth chief executive officer Andrew Cullen after their Championship match with Millwall was postponed because of a power cut.
"I can’t speculate at this time but what we can say is that Fratton Park is served by a sub station and it has been identified that one of the fuse boards had blown," he told BBC Radio Solent.
"The problem is that we don’t have any access to that sub station so our maintenance team weren’t able to get into there to try and resolve the situation themselves.
"We need to do a lot more to find out what caused that to happen, both teams were desperate to play the game so I’m really conscious and upset for the supporters from both teams who made the trip.
"It’s not great and I can only apologise to both sets of supports and those who travelled."
Image source, Rex FeaturesThe game between Portsmouth and Millwall was eventually postponed about 55 minutes after the scheduled kick-off time
Portsmouth chief executive officer Andrew Cullen says they had no option but to postpone Wednesday's Championship match with Millwall because of a power cut at Fratton Park.
"We got to a point where the floodlights were back on but there was still an issue with non-emergency lighting and CCTV so we had a decision to make in terms of crowd safety," he told BBC Radio Solent.
"There’s more to this than just 22 players on the football pitch. We had a capacity crowd here tonight and we did not have confidence that we could get through a whole game with some lighting not working in the stands.
"Then we got to 8:30 and then we had a further problem which was the last trains going back to London for the Millwall supporters, and they would not have been able to get those back so a decision had to be made.
"I’m incredibly disappointed and I’m upset that we had to call the game off but it was the only logical decision at that time to do in the interests of everyone’s safety."
Image source, Rex FeaturesThe last time Millwall and Portsmouth met in a league fixture 12 years ago, a young striker wrong-footed the Pompey defence on the edge of the area before firing low into the corner to secure a 1-0 win for the Lions.
On loan from Tottenham, Millwall's number 37 would go to become a household name and a national icon as his nine goals in 27 games over the second half of the 2011-12 campaign offered a prelude to his greatness.
He was just 18, but Harry Kane's strike to defeat Pompey combined the skill and precision we have subsequently seen again, and again... and again, for club and country.
The England captain has 69 goals for his country - 16 more than second-placed Wayne Rooney - and scored 280 times for Tottenham before moving to Bayern Munich in 2023.
His goal that day at Fratton Park in April 2012 was the last in the league between Millwall and Pompey, with the south coast side relegated to League One that season, and Kane's contribution - including four goals in the final six games - helping steer the Lions to safety.
Pompey's promotion back to the Championship pits the two teams against each other once again, with Millwall on an eight-game unbeaten run and John Mousinho's side just one point above bottom club QPR.
Millwall won both games 1-0 in the 2011-12 season but have won just two of their last 19 league visits to Portsmouth [D7 L10]. The Lions won 1-0 when the clubs met in the Carabao Cup at Fratton Park in August.
Image source, Rex FeaturesJohn Mousinho led Portsmouth to promotion from League One in his first full season in charge
Portsmouth head coach John Mousinho has been impressed with the performances of Wednesday's opponents Millwall.
Pompey host the Lions at Fratton Park having seen their Championship match at Blackburn postponed on Saturday, further delaying their return to action after the international break.
Millwall are unbeaten in their past eight league matches and have only conceded three goals in that run.
"They're well-organised defensively and well-drilled as a team which is something we'll have to work on to try to unpick that lock," Mousinho told BBC Radio Solent.
"We need to move the ball quickly to put them under pressure but while their defensive record is excellent they've also got a real attacking threat as well."
Portsmouth's last game before the break saw them record their first home victory of the season by beating Preston 3-1 and Mousinho is hoping that is a sign of things to come.
"As a general rule you try to build any success on decent home form and when you pick up home wins it gives you a bit more breathing space away which we haven't had so this is definitely a very important game," he added.
