Emergency services have been called to the seafront in Essex after receiving several reports of people coughing and struggling to breathe.
Police, the ambulance service and the fire service are at the seafront off Fourth Avenue in Frinton, after receiving calls shortly after 2pm on Sunday.
Beachgoers said they had been left struggling to breathe but the cause is unknown and is being investigated.
A mother who was on a family day out at the busy beach told how her one of her twin daughters was left “gasping” for breath.
Miriam Lansdell told the PA news agency: “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 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.”
Training manager, Mark Wray, who was enjoying a day at the beach with his wife, said he had noticed a few children coughing as they came out of the sea but assumed they had swallowed some water.
He said a few hours later two men from a beach patrol started going backwards and forwards in a dinghy along a short section of the beach, about 50 feet from the shore, urging people to move back, but that there was no clear instruction to get out of the water.
He added: “Then others, with radios, began patrolling the beach and starting to tape some areas off. It was all very low-key and there didn’t seem to be much urgency to it.
“But as we were heading home a procession of emergency vehicles, including ambulances, fire appliances, police cars and other unmarked vehicles with blue flashing lights started heading towards the scene.”
He said the beach was “packed with hundreds of families enjoying the record-breaking temperatures”.
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.”
Essex Police said that while the cause is investigated, people are advised not to go into the sea.
East of England Ambulance Service said people should wash themselves down if they were in the water, change their clothes and drink fresh water.
Anyone with further concerns is advised to call the NHS on 111.
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.”