An environmental disaster was narrowly averted by quick thinking contractors who prevented hundreds of litres of red diesel from polluting a river.

The oil spill in the River Wandle was reported by engineering contractors who are working at raising the waterway in Wandle Park, on Monday.

Using floating booms the team were able to contain the spill to about 20 metres, stopping the oil spreading downstream and saving wildlife such as ducks, swans and fish from ingesting the toxic fuel.

In 2007, 20 years of river restoration and two tonnes of fish were wiped out from the River Wandle when industrial strength bleach was accidentally released into the water.

Two years later the nine mile river was polluted with red diesel which ran into the river in Beddington Park from a surface water drain.

Swans and ducks had to be rescued from the park after becoming covered in the diesel oil.

As part of efforts to restore the river following the pollution spills, thousands of young fish were released by the Environment Agency (EA) fisheries teams.

They are not thought to have been affected by Monday's incident in the river, which passes through the London Borough's of Croydon, Sutton, Merton and Wandsworth, where it joins the River Thames.

Site manager Pat Boyle of J Breheny Contractors, who have been working on the site for six months, said: "Luckily there wouldn't be any wildlife in distress, as there are no fish in this part of the river.

"We put in booms so if there is any contamination we can control it. It's great we saved it from being worse, it is fortunate we use the booms as part of our methodology of working.

"We alerted the EA to the spillage and contained it further down until they arrived.

"We also dammed the river in two places further down and put four new booms in place which held the scum and the film.

The team were assisted with the clean up by Thames Water Utilities and officers from the Environment Agency.

Peter Ehmann, who is co-ordinating the clean-up for the Environment Agency, said: "It was really good the council contractor's who were doing the work on Wandle Park acted so quickly, or else it would have been far, far worse and caused significant environmental damage.

"We had crews from Thames Water there overnight who collected the oil from behind the booms. We have collected most of the oil now and the source of the oil has stopped."

The EA has launched a full investigation into the cause of the diesel spill, but it is believed the source is somewhere in Croydon.

Mr Ehmann added: "The oil is unused so there is a value to it, this makes fly tipping unlikely but it is a line of enquiry we are considering."

Croydon Council's pollution team are liaising with the EA if any help is required to identify the source.

Members of the public with any information on the incident can call the Environment Agency's 24 hour hotline on 0800 807 060.