Campaigners said they would ‘steer very clear’ of a lake in a country park that has turned green.

Witney Lake is blanketed with a bloom of blue-green algae which the town council said posed "potential health risks to both humans and dogs".

At the end of August health warning notices were put up at the lakeside, advising residents to "take appropriate precautions" and not allow their pets to swim in or drink the water.

Last Tuesday (September 17) the council said the lake has been inundated for a number of months this year from flood waters from the River Windrush.

(Image: Eve Smith)

“This has raised the level of potassium and other contaminants which has resulted in its current algal bloom.

“While the lake is safe to walk around, please protect yourself and any pets by not swimming or having any contact with the water.

“This type of algae can pose serious health risks therefore it is important to take the appropriate precautions."

It added that the council team was regularly at the lake to make residents aware of the issue.

The Environment Agency said its team had "no evidence of pollution or harm to wildlife".

But West Oxfordshire-based rivers campaigners Windrush Against Sewage Pollution (WASP) said they had seen similar at nearby Dix Pit caused by discharges from Witney Sewage Treatment Works.

Chair Ash Smith said: "How dangerous it is will be a matter for expert analysis keeping track of how it develops.

"I would steer very clear of it having seen the devastating effect on Dix's Pit a few years ago, where blue-green algae formed, we believe from our investigation, caused by leachate from a landfill site but unconvincingly written off by the Environment Agency as 'natural'.

"It killed thousands of fish."

He added: "I predicted this would happen when I filmed the Windrush running into the lake in March after we had so much untreated sewage dumping going on, carrying phosphate nitrates and organic matter into the lake to sit there and wait for conditions to be right to feed an algal bloom.

"We could see brown algae in the river even then."

Lidia Arciszewska, West Oxfordshire District Council's executive member for environment and an Oxford University biochemist, agreed sewage pollution could not be ruled out as a cause.

She said: "Algae blooms can occur when a combination of suitable environmental conditions exist for abundant algal growth - for example, increased nutrients, warmer temperature, abundant light, and stable wind conditions - which does reflect the current weather conditions we are experiencing.

"However, obviously such growth may reflect pollution of the Lake with nutrients rich material.

"The Colwell Brook, which receives the discharge from the Witney sewerage station, flows very close to the Lake where it runs parallel to the A40.

The EA said the 'naturally occurring' blue green algae was caused by warmer temperatures​ (Image: Eve Smith)

"Without any further investigation we can't absolutely exclude a possibility that some spillage from the Colwell Brook has got into the lake at some point, but it would need to be a frequent occurrence to create sufficient nutrients to promote a large algae bloom across the Lake.

"We are not aware of any outfalls into the Lake which may have mis-connections from other sources in the area. Witney Town Council, the Lake owners, are in better position to assess this."

A spokesperson for the Environment Agency said its specialist officers took water samples from Witney Lake at the start of last week and have since confirmed the presence of naturally-occurring blue-green algae, caused by warmer temperatures.

He said: "We’ll continue to liaise with the council until the algae disappears.

“Anyone with new concerns, or who suspects pollution in our waterways, can contact our 24-hour incident hotline on 0800 807060.”

Thames Water has been contacted.