Thames Water is urgently investigating how high levels of ammonia ended up in two Witney streams - killing 1,700 fish.

The company said someone had deposited water containing high ammonia levels into the sewage system.

The water then flowed through its treatment works in Witney and into Colwell Brook and Queen Emma's Dyke next to Witney Lake.

Thames Water staff and officers from the Environment Agency are on site and working to find the source of the ammonia.

Witney Town Council said they had been made aware that the EA has dammed the dyke upstream while investigations continue.

The council said the lake is self-contained and no water feeds directly into it but that people should take precautions anyway.

Thames Water spokeswoman Sarah Sharpe said: "We’ve identified high ammonia levels in the stream which we believe are due to discharges from our sewage treatment works.

"We’re investigating what has caused these discharges to contain such high ammonia levels, most likely substances being put into the drainage network which shouldn’t be there.

“Our specialist teams are urgently analysing samples to find out what has gone into the water, and we’re continuing to support the Environment Agency in their work to protect the stream from further damage.”

An EA Spokesman said: "The Environment Agency is investigating the death of approximately 1,700 fish on a 2km stretch of Colwell Brook and Queen Emma's Dyke near Witney Lake in Oxfordshire.

"Investigations are ongoing and Environment Officers are on site, taking measurements from local rivers and conducting a fish survey, with an ecology survey planned for tomorrow".

No official warnings have been released about water quality.