AN explosion which injured and almost killed four firefighters is being treated as arson by police.

The blast happened at a derelict scrapyard in Farnworth — and the fire service said four firefighters could have been killed if the blast had happened 30 seconds earlier.

All the men were in their 40s and have more than 80 years’ experience in the service between them.

An acetylene cylinder exploded as the firefighters were using ladders to scale a fence next to the scrapyard, off Wellington Street, to get nearer to the fire.

They were taken to hospital for treatment to minor injuries, and some of them suffered a temporary loss of hearing because the sound of the explosion was so loud.

People living at least a mile away from the explosion reported feeling the force of the blast.

Police now believe the fire, which happened at 1am on Saturday, was started deliberately and are investigating.

Det Insp Gary Smith, of Bolton CID, said: “This is currently under investigation and we are urging anyone with any information to contact us.

“Whoever has done this is playing with danger, not just to themselves, but other people, too.

“This prank could have been deadly.”

The firefighters had just climbed down the ladders when the cylinder exploded and smashed all the windows in the building.

The shocked firefighters said they had never seen anything like it before.

Whenever firefighters deal with a fire in which it is suspected acetylene cylinders may be involved, a cordon is set up around the building to protect the public and the blaze often has to be cooled with water for several hours to make sure it is safe.

The fire service is appealing to people to dispose of acetylene cylinders responsibly and said the consequences of failing to do so could be disastrous.