The Friends of Richmond Park have spoken of their determination to help in the clean-up operation of Beverley Brook which has been hit by a disastrous oil spill.

The Brook runs through the Royal park and has been at the centre of a restoration project supported by the charity and its patrons, who include Sir David Attenborough and Clare Balding.

However, the brook was polluted over the weekend with a spill, thought to include diesel, which has been traced to a tributary upstream at Pyl Brook in Morden.

Alarmed walkers have posted images of the thick black oil on the surface of the water.

Thames Water brought in a team to try and limit the spill by putting up booms across the brook, however the pollution had already spread for some distance.

Jade Johnson posted images from Richmond Park, writing: "Oil spill's leaked down into Richmond Park - never seen anything like this."

The Environment Agency is trying to establish who was responsible, while the South East Rivers Trust, which has been involved in regenerating the waterway, is also involved in a rescue operation.

The chairman of the Friends of Richmond Park (FRP), Roger Hillyer, told Nub News: "This is sad news for the wildlife who depend on Beverley Brook and visitors who have enjoyed Its natural beauty.

"The Friends will be working closely with The Royal Parks, the Environment Agency and the South East Rivers Trust to understand the extent of the damage and how we can support the clean-up operation at Beverley Brook."

Dog owners have been advised to keep their pets out of the brook, which is home to several fish species and other wildlife.

It is not yet clear how much damage has been done and how long the clean-up will take.

One workman on the site described the diesel spill as the worst he had seen while an eye-witness described the smell as putrid.

A spokesman for the South East Rivers Trust said: "We always find it shocking when pollution incidents occur on rivers.

"The Pyl Brook and Beverley Brook are urban rivers treasured by the community and an important resource for wildlife. As a charity, we have carried out river improvement work along the Beverley Brook through both Richmond Park and Wimbledon Common.

"We are very sad to hear that the Brook and Thames have been affected.

"We know that Thames Water and the Environment Agency have been working hard since the incident began to try to clear up the pollution and to investigate its source."

Thames Water, which is leading a clean-up effort, told Nub News: "We're aware of an oil pollution in Beverley Brook and have been working with the Environment Agency and contractors to minimise it's spread and impact to the river.

"Our teams are currently tracing and establishing the source of the pollution, which we believe was caused by a third party, and we have deployed booms and pads to help prevent the oil further spreading in the watercourse."

Beverley Brook has been the focus of a restoration project led by Sir David Attenborough and financed by the Royal Parks charity along with key partners including the South East Rivers Trust, Friends of Richmond Park and the Environment Agency.

When the restoration scheme was launched, Sir David, who is patron of the Beverley Brook Restoration Project, stressed the importance of the work, saying: "Projects like this that restore natural habitats are vital to ensure our riverside flora and fauna thrive in the future."

An Environment Agency spokesperson said: "We were informed on Sunday afternoon of oil pollution in the Beverly Brook which flows through Richmond Park.

"Our officers went onsite with Thames Water to trace the source and also assess the impact of the pollution. Thames Water have begun the clean-up operation and we will continue to monitor the situation.

"Members of the public should report pollution incidents to our free 24-hour Incident Hotline on 0800 807060."

The brook is a popular attraction in Richmond Park for visitors and photographers.

Its name is derived from the Old English for Beaver and Stream, and means a place where beavers live – though no beavers have been able to make their home there for many years, not least because of urban pollution.