Two more bodies have been found on the beach at Camber Sands, East Sussex, where three men died after being pulled from the sea earlier on Wednesday, the RNLI said.
The men's bodies were discovered at around 8pm by a member of the public as the tide receded, taking the death toll at the popular tourist beach to five.
A search is under way for a further missing person, with two lifeboats and a helicopter combing the water off the coast.
Richard Tollett, lifeboat operations manager at Rye Harbour, told the Press Association the RNLI received a request to launch at around 8.30pm.
He said: "As the tide has receded it has left a couple more bodies on the beach in the sand at Camber.
"A member of the public found them and that person reported another one in the water so we have got two lifeboats and a helicopter searching the area trying to find the other one."
The Maritime and Coastguard Agency said coastguard rescue teams from Rye Bay and Dungeness were also involved in the search, along with a coastguard helicopter from Lydd and Sussex Police.
The first three men died despite efforts by medics to save them after being pulled from the sea near Rye at about 2.15pm.
Suggestions that their deaths may be linked to jellyfish have been ruled out, and police said there was nothing to suggest the men were migrants.
Chief Superintendent Di Roskilly said: "This has been an incredibly tragic situation. At this stage we are doing all we can to establish who the men are and to identify next of kin."
Police remained on the beach during Wednesday - the hottest day of the year - as day-trippers said they were surprised there were no lifeguards present.
One beach-goer said police told her to stay out of the sea due to a rip tide while another, who declined to be named, said: "We noticed when we came here that there were no lifeguards. We had kids here and we were worried about them."
He said he could not understand why the men had run into difficulties as the sea appeared calm to him.
He said: "The sea is very shallow for quite a long way. It seems so strange how they got into trouble. There was no waves and no wind."
Eyewitness Natalja Taylor, 30, who was on a day trip with her husband, said police were driving up the beach with a loudhailer urging people to stay out of the sea.
She said: "We were sat on a hill a bit further away and we saw three people being pulled out of the water. I think the people who rescued them were regular people, not emergency personnel.
"We don't know what condition they were in. There were so many people there. They were still on the beach when we left.
"Police drove on to the beach with a loudspeaker, telling people not to go into the water until further notice. They also taped off a huge chunk of the beach so no-one could get near it.
"It's pretty scary. I'm not sure how it happened. We decided not to go into the sea after seeing warning signs about some kind of fish. It wasn't particularly windy down there, it was just a hot, sunny beach day.
"There was a lot of people on the beach, particularly for a weekday. We're now just hoping that everyone involved is okay."
It is the second serious incident to take place recently at Camber Sands, which with its sand dunes has long been a popular destination for beach-goers.
Last month 19-year-old Brazilian Gustavo Silva Da Cruz died after getting into difficulty while swimming in the sea there.
Mr Da Cruz, who was visiting family in Croydon, south London, was one of three men who got into trouble. The two others, who were not connected to him, included a man aged 35 and his son aged 17.
The five who perished at Camber Sands bring the death toll around Britain's coastline to 12 over the last week, prompting calls from the RNLI to urge seaside visitors to take care and respect the water.
Two-year-old Mckayla Bruynius died at Bristol Children's Hospital on Tuesday night after she was caught by a large wave at Fistral beach in Newquay, Cornwall, on Friday.
Her father, Rudy Bruynius, was also killed after getting into trouble as 13ft waves lashed the coast amid strong winds and rain.
A mother and son died on Saturday after a rescue operation at Aberdeen beach, while a windsurfer in his 60s died in a Colchester hospital after being rescued off the coast of West Mersea, Essex.
On Sunday a woman in her 30s died after getting into difficulties while swimming off Jersey, before a man died despite the efforts of medics after getting stuck in a rip current at Sandbanks beach in Poole, Dorset.
Witnesses told The Sun that the first three men recovered in Wednesday's incident were fully clothed.
There are no lifeguards at the beach but a spokesman for Rother District Council told the BBC that in the summer beach patrols were provided to advise people of potential dangers, reunite lost children with their families and deal with incidents on the beach.
"While it's very upsetting to see two similar, tragic incidents this summer, over the years these kind of incidents are extremely rare and on a fine day around 25,000 people use the beach safely," he said.
"Although it's too early to draw any conclusions from this latest incident, in recent years we have seen a change in the make-up of visitors to Camber, including more people from outside the area who are not familiar with the sea and the dangers it can pose."
An online petition started a month ago to campaign for lifeguards on the beach has reached more than 3,600 signatures.