Plumes of black smoke were spotted coming out of a disused railway tunnel after a pile of rubbish was set alight.
Firefighters were called to the incident after smoke was seen at an entrance to the Miley Tunnel in St Peter’s Square, Preston at around 6.20pm on Monday.
Graham Nelson, crew manager, said: “It was in the old railway tunnel between Great George Street and St Peter’s Square.
“Whoever had done it had broken a bit of fencing to get in and climbed over the wall.
“There was a lot of smoke billowing out at the St Peter’s end with the wind direction so we went to the other end to gain access.”
He said a crew of firefighters were at the scene for around half an hour.
Mr Nelson said: “It had died down by the time we got in there.
“They had set fire to some rubber tubing, which was causing lots of black smoke.”
The single track railway tunnel connects Maudland Bank and Great George Street.
It forms part of the abandoned Longridge to Preston Railway route and leads to a disused half-mile stretch, which runs between its entrance and Skeffington Road and has been earmarked for Preston’s new tram line.
It is a hotspot for fly-tipping and a 2011 clean-up, undertaken by Network Rail in conjunction with Preston Council, produced an estimated 10 tonnes of rubbish.
A collection was filmed as part of the BBC series Filthy Rotten Scoundrels, focusing on environment officers’ work.