FIRE chiefs have launched an urgent investigation after children risked setting off a potentially lethal timebomb by starting a fire in a disused mill packed with discarded gas cylinders and tonnes of timber.

The first floor of the mill, in Aqueduct Street, Blackburn, was set alight by youngsters believed to have been playing with matches shortly after 9pm yesterday.

But firefighters were horrified to discover that the four-floor building contained thousands of tonnes of timber waste and dozens of discarded gas cylinders.

Now they have appealed for Blackburn with Darwen Council to clear and secure the building immediately.

More than 25 firefighters from Blackburn, Darwen and Accrington tackled the blaze until early today.

But Blackburn Fire Station Officer Tom Davies said the youngsters, who have not yet been identified, were lucky to be alive. He said had the timber and gas cylinders caught fire the building would have been destroyed and the lives of nearby residents put at risk from explosion.

He is now trying to trace the owners of the disused building, formerly Cardwell Mill, which is in the hands of Manchester estate agents King Sturge and Co. A spokesman for the firm said they were looking into the matter.

"When we arrived, discovered the mill stacked full of timber rubbish and gas cylinders," he said.

"It was in a dangerous state and highly combustible.

"It was terrifying. We believe the fire was started on the first floor by children.

"The fire took hold very quickly and, had the youngsters been on an upper floor, they would not have been able to get out.

"And if the gas cylinders had ignited we would have had a major incident on our hands."

He added that in 23 years in the fire service he had never come across a building in such a dangerous state.

Blackburn Police are investigating.

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