Harbour master Steve Bassett issued the warning as he has seen the number of people swimming with seals in St Ives, Cornwall, increase over the last few years, and he fears for children’s safety.
“I’m afraid that some day someone will lose an arm,” he said.
“If seals are both going for territory they can be extremely aggressive.
“And when a hand comes out towards them they will think that’s food.”
St Ives is home to a 40-strong seal colony which lives on rocks just a few miles from the resort.
Bassett explained that the seals are being drawn to the harbour by the prospect of food.
“God forbid something should happen, but one day a seal is going to feel trapped and bite back,” he said.
“Seals are massive and the power they have in their jaws is unbelievable.”
Gill Bell, head of conservation at the Marine Conservation Society in Wales, said wild creatures should not be interfered with.
“You would not allow a child to approach a wild dog with food, so why allow them to approach a seal?,” she said.
“Seals are like toddlers, they will put anything in their mouth.”
Bell added that seals carry diseases that are highly infectious to humans, so even a small bite could cause serious harm.
“You should never swim over to where they are because that’s when you get issues,” she said.
“The main concern is that they could misinterpret an action as a supposed threat.
“They are very gorgeous to look at and that’s what we should be doing, looking at them and not getting close to them.”
Bassett has now put up notices around the harbour warning against approaching or feeding the seals.