Police have warned bathers in parts of Essex not to go into the sea after reports of people coughing and struggling to breathe.
A 'large number of people' have reported breathing difficulties after attending Frinton and Clacton beach this afternoon.
The East of England Ambulance Service (EEAST) has said that patients have found their symptoms have improved after coming out of the water.
A triage point for those with breathing difficulties has been set up at Walton pier, according to people who had been on the beach this afternoon.
A statement from Essex Police said: "Emergency services have received reports of a large number of people suffering from coughing on the seafront in Frinton and Clacton this afternoon.
"The cause for this is currently unknown and we and our partner agencies are working to try and establish the cause as quickly as possible.
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"While we are doing this, we are advising people to not go into the sea."
Mum-of-two Miriam Lansdell said: "My daughter started coughing. She said 'I don't feel good. It hurts to breathe in'.
"My other daughter was gasping and couldn't form words because she couldn't breathe well enough."
The 45-year-old mental health worker from Derbyshire, who was visiting her parents in Essex for the Bank Holiday weekend, said she had also had difficulty breathing as she lay on the sand drying off after a dip in the water.
She said they all began to breathe easier when they moved further away from the beach, but took the 10-year-old win girls to a walk-in clinic to be checked over by medical staff.
Ms Lansdell said her father had been told by someone in a speedboat, who he assumed to be associated with the coastguard, that there may have been a fuel spill.
She said: "My dad said he had been asked to get out of the water by a man on a boat. He asked why and the man said there had been a fuel spill. He said if anyone is having breathing difficulties they should probably call an ambulance."
She added: "It's not what you expect when you go for a day out to the beach."
Speculation online of a fuel spill has not been confirmed by police or the ambulance service.
One person tweeted that there were "lots of people coughing heavily", while a mother said her son began coughing after swimming and had to be given his inhaler.
Another said: "We have just left Frinton and have seen lots of fire engines on the way out. Has there been an incident? We were on the beach and all developed a cough and were struggling to breathe."
A spokeswoman for East of England Ambulance Service said: "We are aware of an incident on Sunday 25 August with reports of a number of people suffering from coughing on the seafront off Fourth Avenue, Frinton.
"We are assisting the police and fire services with this incident. The cause is currently unknown."
