Residents fear raw sewage may have been leaking continuously into an Oxford street for more than a year.

Mystery surrounds the leak, in William Street, Marston. The water was initially thought to be from a spring, but Thames Water and Oxford City Council officials remain baffled as to where it is coming from.

But members of the New Marston Residents Association said they were told by workmen from the water company that tests revealed ammonia in the liquid -- suggesting it could contain raw sewage.

Thames Water has denied the claims. They insist the leak is surface water, though worried residents have enlisted the help of Oxford MP Andrew Smith, who has written to the water company about the problem.

Allison Ozkaya, 38, of William Street, said she was worried for the safety of her two-year-old daughter and other children who play near the leak.

Colin Peach, 51, also of William Street, and his wife Linda, 52, have been worried about the problem since it began last April.

He said: "I have written to Mr Smith and he shares our concern.

"This problem started last April and since then the road has collapsed twice.

"Every so often workmen come and look at it, scratch their heads and go away again.

"Until Thames Water sorts it out I certainly won't be paying the amount of my water bill that is meant to deal with sewage."

Susan Mallett, of the residents' association, said: "Workmen tested the water and told me it was full of ammonia, which means it is sewage. The road is about to subside for the third time."

The Oxford Mail reported in February that neither Thames Water nor Oxford City Council could find a cause for the leak, but had pledged to work together to find a solution.

Thames Water spokesman Andrew Boyd contradicted residents' claims. He said further tests showed that the water containing ammonia was not sewage, but surface water.

He said: "We have had a meeting with Oxford City Council and concluded it is ground water, coming from the soil."

City council spokesman David Penney said the officer investigating the problem was on holiday, and would not be able to explain how he planned to solve the issue until next week (beginning May 28).

Mr Smith was not available for comment.