The Environment Agency was today investigating suspected water pollution in Chorley.
Agency officials posted a picture on Twitter which appeared to show an oily substance discolouring the River Chor in Astley Park.
This afternoon it was revealed that the likely source was a drain from the nearby M61 motorway.
Thomas Charlton from the Environment Agency, said: “This morning (Tuesday 4th December), the Environment Agency received reports from the public that the River Chor at Astley Park had an odour of petrol and was coloured with what appeared be an oil substance with a rainbow sheen.
“Officers from the Environment Agency attended immediately to find the source of pollution, stop it, and reduce the impact on people and the environment.
"The pollution was visible through Astley Park up to junction 8 of the M61, where the pollution has been traced to a highway drain from the motorway.
"Environment Agency officers placed an oil absorbent boom across the watercourse to reduce the impact on the environment and protect the water quality.
“We believe at this stage the heavy rain during the night may have flushed out the pollution from the drain.
“The Environment Agency would like to thank everyone who reported the pollution to their incident hotline on 0800 80 70 60.
"This enabled us to respond quickly to protect people and the environment and reduce the impact of this incident.”