Emergency services have scrambled to a seafront in Essex after receiving several reports of beachgoers coughing and struggling to breathe on Sunday.
Police, the ambulance service and the fire service were called to the seafront off Fourth Avenue in Frinton shortly after 2pm.
Beachgoers said they had been left struggling to breathe but the cause is unknown and is being investigated.
Essex Police are now warning people to avoid going in the sea, with the force saying patients have found symptoms improved out of the water.
Speculation online of a fuel spill was not confirmed by police or the ambulance service.
A mother who was on a family day out in Frinton told how her one of her twin daughters was left "gasping" for breath.
Miriam Lansdell, from Derbyshire, 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 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 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.
"It's not what you expect when you go for a day out to the beach."
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.
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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."
Tweeting Kent Police, one person said: "Something is not right at Frinton-on-Sea, 80 per cent of kids that have been in the water are all coughing very bad."
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."
Additional reporting by Press Association.
