An LUL investigation is under way into how and why the contractor - one of the firms contracted to build the East London Line extension - took what amounted to two massive potential bombs to track-replacement works on August 12.

"Balfour Beatty appear to have broken just about every safety rule in the book by taking these potentially explosive cylinders onto the Victoria Line for 48 hours," RMT general secretary Bob Crow said today.

"Acetylene is extremely flammable, unstable and explosive, and is only allowed onto the Underground in half-litre containers under strict rules, and it seems that Balfour Beatty flouted them.

"We understand they had no hot-works licence, no permission to take the cylinders underground and no method statement on transporting them - and that this was not the first time they had done it.

"If that was not bad enough we understand they even transported four oxygen cylinders alongside the acetylene - a potentially catastrophic mixture.

"This incident underlines the case we have been making that we cannot afford to lose the Section 12 sub-surface fire-safety regulations that lay down strict rules for what can and can't be done down there.

"The idea of giving employers the initiative on fire-safety sends a shiver up my spine, because where there's a profit to be made the privateers simply can't be trusted, and the sooner all Tube infrastructure work is brought back in-house, the better," Bob Crow said.