We've now ended our live coverage - you can follow the latest updates in our story
A Palestinian couple and their teenage daughter who died after entering the water off Shoreham, West Sussex, have been named by police
They are Hassan El-Khawas, 55, Zeina Alayn, 37, and 14-year-old Sara El-Khawas - here's what we know about the family
The Palestinian Forum in Britain says a girl who is in a critical condition in hospital is the couple's younger daughter and that two other children were on the shore at the time of the incident
The forum says the family were British citizens of Palestinian origin whose extended family had lived in the UK for many years
The family lived in London and travelled to Shoreham, Sussex Police say. The force also says it is working with the coroner "to understand exactly how this incident occurred"
Prime Minister Andy Burnham calls it "a real terrible family tragedy" and warns about "the dangers of open water swimming"
A spokesperson for Shoreham Port says it is "working with multiple agencies regarding this incident", which emergency services were called to at about 15:00 BST.
It added that it "would urge the public to avoid the area".
Kingston Beach, where the incident is taking place, is a short distance from Shoreham Port on the West Sussex seafront, near Shoreham-by-Sea.
The town is about a 20-minute drive away from Brighton.
James Kelly
BBC News
We've been hearing from a nearby resident, Pete Book, who says he was "walking home from the pub" when he "saw and heard the Coastguard helicopter circling over the lifeboat station on the beach".
"As I neared home, a yellow air ambulance helicopter landed. There were a lot of ambulances, Coastguard and police vehicles converging on the area, and the coast road was closed," he says.
Book adds: "An orange-and-white air ambulance then landed, and the Coastguard helicopter flew off."
He says the road is still closed, and there are still "lots of emergency responders and helicopters at the scene".
"The lifeboat had also launched and was being returned to the station."
The A259 is closed in both directions due to the emergency services incident, between New Road and Kingston Lane.
The street is a main commuter road between Brighton, Shoreham and Worthing on the South East coast, and further into West Sussex.
Helicopter footage above Shoreham Harbour shows the size of the emergency response after services were deployed following reports of four people getting into difficulty in the water earlier this afternoon.
A number of ambulances are on the scene, along with several police vehicles.
Tom Rutland, the Labour MP for East Worthing and Shoreham, has urged people to "stay clear of the area around Shoreham Fort and Kingston Beach as emergency services respond to reports of several people in difficulty in the water there".
In a post on X, he says that "the incident remains ongoing".
Helena Wilkinson
Speaking to BBC Radio Sussex
It is very clear that this is a huge emergency operation taking place in Shoreham.
We have had a number of statements from emergency services including Sussex Police. It is very clear that it is a massive emergency service response with a number of agencies supporting the operation.
What we don't know is the kind of difficulty the four people are in, and no information has been released in terms of how far out to sea they were, or the current condition they are in.
A spokesperson for the Air Ambulance Charity in Kent, Surrey and Sussex (AAKSS) says it sent both of its helicopters and critical care team to the scene of the incident today "as part of the coordinated emergency response".
As a reminder, emergency services were called at about 15:00 BST.
There is a large emergency services presence on the seafront in Shoreham, West Sussex, this afternoon after four people got into difficulty in the water.
Information is still coming in, here's a look at what we know - and what we don't - at the moment.
What is unclear at the moment is what kind of difficulty the four people are in. No information has been released on how far out to sea they are, if they have any sort of vessels or floatation aids with them, or the current condition any of them are in.
We will continue to keep you updated throughout the evening.
The Coastguard says it "responded to an incident reported at Shoreham at about 2.45pm".
"An HM Coastguard helicopter was sent as well as Littlehampton, Shoreham and Newhaven Coastguard Rescue Teams, and an RNLI lifeboat from Shoreham," the spokesperson adds.
"Sussex Police and South East Coast Ambulance Service also attended."
Image source, Eddie MitchellBBC News' Callum French is in the area and says he saw helicopters flying overhead and ambulances heading towards Shoreham Port.
"There was a lot of traffic and they closed the road for a bit," he said. "It's the biggest turnout I have seen in the last few years here."
He added that, despite recent warm weather, the sea conditions have been "pretty rough" due to the high winds.
The incident took place in Shoreham Harbour
We now have more from South East Coast Ambulance Service, which says it was called to "reports of four people in need of medical attention after getting into difficulty in the water near Shoreham Harbour".
"We were called shortly after 15:00 BST and have a number of ambulance resources at the scene," it added.
Sussex Police says that emergency services were called to reports of four people in difficulty in the water close to Shoreham Port this afternoon.
"A section of the A259 in Shoreham has been closed as the incident remains ongoing, and we’re urging the public to stay clear of the area in the meantime," added police.
An RNLI spokesperson says: "Shoreham RNLI were tasked by HM Coastguard at approximately 3pm on Tuesday 18 August."
We can now bring you more images from the scene, where police and search and rescue crews are dealing with the incident in Shoreham Harbour this afternoon.
We will bring you more information as soon as it becomes available.
Image source, Eddie MitchellA spokesperson from the RNLI says on its website that lifeboat crews were launched at about 14:50 BST on Tuesday afternoon, though it is not confirmed what the nature of the incident is, or whether the lifeboat launch is linked.
Two other lifeboats were also launched from the Brighton lifeboat station shortly after 15:00.
Several ambulances from South East Coast Ambulance Service are dealing with the incident this afternoon, alongside coastguard helicopters and other search and rescue vehicles.
Emergency services have been approached for more information.
Image source, Eddie MitchellAmbulances and an air ambulance in Shoreham, West Sussex
Thank you for joining our live coverage as reports are coming in about a large emergency services presence on the seafront in Shoreham, West Sussex, this afternoon.
A number of helicopters, ambulances and other emergency service vehicles are at the scene. We will bring you more updates as we have them.