A group of swimmers in Wolvercote, Oxford, had to cancel their Boxing Day dip after Thames Water informed residents of a “sewage release” on Christmas Day.

The email, seen by the PA news agency, confirmed an “ongoing sewage release” at nearby Whitney, adding: “If you’re thinking of entering the river, please remember that it can take up to four days for the sewage to clear.”

The email also said: “Putting untreated sewage into rivers is unacceptable to us, but after heavy rain it’s sometimes necessary and permitted.”

Dr Fiona Palumbo Tolan, a 42-year-old academic who is part of the group, told PA she was “extremely disappointed” by the incident.

“We were extremely disappointed not to be able to swim today.

“Every week we have to check for notifications of sewerage releases before being sure that it’s safe to swim”, Dr Palumbo Tolan said.

“Personally, I find it deeply distressing that the river is made inaccessible because of pollution from sewerage.

“It seems that what should be a very last resort in the case of extreme emergency has become a very common tactic of convenience for the water companies.”

Thames Water and the Department for Environment, Food and Rural Affairs have been approached for comment.