Sewage has now been leaking into Oxfordshire waterways for five days and the spill could continue until tomorrow.

A tweet from the alert system on Wednesday morning said the release, which began from Witney Sewage Treatment Works, in the early hours of Christmas Day, could continue until Thursday.

The alerts, which are tweeted via @oxthamessewage, cover six sites and are posted by water company Thames Water, Oxford City Council and charities Thames 21 and The Rivers Trust.

Treatment works are only allowed to put sewage into waterways after wet weather and when they are close to capacity.

On Sunday, a group of swimmers in Wolvercote had to cancel their Boxing Day dip due to the releases.

In an email sent to the group and seen by the PA news agency, Thames Water advised that "it can take up to four days for the sewage to clear.”

Thames Water has accumulated £32 million in fines for crimes relating to water pollution in the Thames Valley area.

Ashley Smith of Witney Against Sewage Pollution (WASP), a group of volunteers, investigating the pollution of the River Windrush and her sister rivers, said: “We have been lucky so far his winter but now the rain is with us and the sewage spilling season has started.

“These should be short events but the last two years has seen them go on over months and get worse. Every 100 new houses will mean about an extra 30 tonnes of sewage a day that will be going into the Windrush and Thames untreated during these spills.”

Thames Water apologised for the cancellation of the Boxing Day swim.

A spokesperson said: “It’s our view that discharges of untreated sewage are simply unacceptable, even when they are legally permitted, and we’ll work with the Government, Ofwat and the Environment Agency to accelerate work to stop them being necessary.

"We have an unprecedented amount of investment directed towards safeguarding our rivers and streams. Between 2020 and 2025 we are spending £1.25 billion on maintaining and improving our operational sites, including contributing to the health of 745km of rivers across London and the Thames Valley.

"We have a long way to go – and we certainly can’t do it on our own – but the ambition is clear.”

In a statement the Environment Agency said its staff respond to pollution incidents 24/7 and that water companies are legally required to report any breaches of their permits.

“We will always seek to hold those responsible for environmental harm to account. Specific action will depend on the nature of the offence, up to and including prosecution," it said.