Around 150 people said they felt unwell after visiting a beach in Essex over the bank holiday weekend.
Sun-seekers reportedly suffered vomiting and breathing difficulties at beaches close to Frinton-on-Sea, Clacton and Walton-on-the-Naze.
Fourteen people were hospitalised at nearby Colchester General after being in the water.
According to the BBC, however, no-one was seriously ill.
A spokesperson for Tendring District Council, which oversees the three beaches, said the cause of the symptoms was unknown.
Essex Police said they were investigating, but advised people not to enter the water.
Emergency services, including the ambulance and fire service, attended the scene at the seafront off Fourth Avenue shortly after 2pm.
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Despite speculation of a fuel spill from a ship, the Maritime and Coastguard Agency, which sent a counter surveillance pollution aircraft to the scene, said no evidence of this has been found.
A mother on a family day out at the busy beach, where temperatures reached a high of about 28C, described how one of her twin daughters was left “gasping” for breath.
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 more easily 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 Landsell added: “It’s not what you expect when you go for a day out to the beach.”
A spokeswoman for the Maritime and Coastguard Agency said: “At around 2.30pm on Sunday afternoon, HM Coastguard received reports of beach swimmers suffering from a possible respiratory irritant causing people to cough at Frinton and Walton-on-the-Naze on the Essex coast.
“The Maritime and Coastguard Agency (MCA) has not received any reports of pollution being spilled in the area from vessels.
“The MCA’s counter pollution surveillance aircraft has flown over the area this afternoon to undertake an aerial survey and no fuel pollution has been reported by this flight.”