THE company which will extend the East London line has been accused of putting passengers at risk when it took what a union says amounted to "two massive potential bombs" on the Tube.

Transport union RMT said engineering company Balfour Beatty took a "massive and unacceptable gamble with people's lives" by taking two 54kg containers of highly flammable and unstable acetylene gas into the Victoria Line.

The cylinders were taken into the Tube during track replacement works on August 12, between Kings' Cross and Seven Sisters.

RMT general secretary Bob Crow said: "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.

"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."

The company is a part of Metronet, which is one of the two private infrastructure consortiums responsible for maintenance of London Underground lines.

A Metronet spokesman said it was investigating the claims.

A spokesperson for London Underground said Balfour Beatty had not breached any safety rules and the necessary safety permits were obtained.

He said no investigation was being undertaken by LU.

"Acetylene gas and oxygen are essential materials for track cutting and welding and can only be used during engineering hours or weekend closures when passenger services are suspended. There are strict safety regulations for their use on the Tube.

"Therefore there can have been no risk to passengers because none were present."

Balfour Beatty also maintained the Hatfield site where a cracked section of track caused an express train to derail in 2000, killing four people. Manslaughter charges against the firm were thrown out, but it pleaded guilty to breaching safety standards.

Earlier this week, the company was awarded the £363 million contract to extend the East London line, which is a key in London's Olympic preparation It will help fit six miles of track and build four new stations to create the East London Railway (ELR).

Due to open in June 2010, the ELR will extend the existing East London Tube line to Dalston in the north and West Croydon and Crystal Palace in the south.

Along with four lines in north London, it will form part of the "London Overground" - a new Metro-style service in outer London with trains as frequent as on the Tube.