Hopes of averting a strike by thousands of London Underground workers were dashed today when union leaders confirmed that the walk-out will go ahead tonight, threatening travel chaos for millions of passengers.

The Rail, Maritime and Transport union said more than 2,300 of its members walked out at 6pm for 72 hours, threatening massive disruption to the Tube system.

The workers, who maintain tracks, trains and signals on most of the Tube network, including some of the busiest routes such as the Victoria, Central and District lines, are embroiled in a row linked to the collapse of maintenance giant Metronet.

General secretary Bob Crow said the strike will go ahead after the company and its administrator failed to give the "unequivocal guarantees" on jobs, transfers and pensions that the union is seeking.

A second 72-hour strike is scheduled to start at the same time next Monday, September 10.

"We have been seeking simple, unqualified guarantees from Metronet and its administrator that there will be no job losses, forced transfers or pensions cuts, and we have not had them.

"The efforts the Mayor and Transport for London have put in to try to broker a deal have been welcome, but the problem for all of us remains that Metronet and its administrator are the employer, and the qualified assurances they have given cover only the period of administration.

"It is astonishing that the administrator can decide all sorts of things, including who will take over the PPP contracts, but is unable to give an unequivocal guarantee that the jobs of the people who will actually deliver the Tube’s upgrades will be safe.

"We have been told that the pension-fund trustees will be ’urged’ to ensure that employees lose no pension during the period of administration, but no amount of ’urging’ amounts to a guarantee, and this is not a matter for the trustees in any case.

"It is the employer’s duty to ensure that pension provision will be no less favourable than before the PPP, as promised by the Deputy Prime Minister and what we need from the employer is the simple guarantee that there will be no reduction in pension rights, past, present or future.

"We said from the start that our members were not prepared to pay for the collapse of Metronet with their jobs and pensions, and that remains our bottom line.

"What our members want is to be transferred to a public-sector organisation, and that is the only way in which their jobs and pensions can be protected."

Transport for London (TfL) said all three Tube unions had received assurances from Mayor Ken Livingstone, the administrator and Metronet over the concerns they have raised about jobs, transfers and pensions.

"The administrator and Metronet have made clear that there will be no job cuts, no transfers and that pensions will be fully protected while the company is in administration," a TfL spokesman said.

Mr Livingstone said the unions had been assured there would be no job losses, transfers or loss of pensions as a result of the collapse of Metronet, adding: "It would be incomprehensible to disrupt the lives of millions of Londoners and lose their members significant amounts of pay when all of the assurances they have asked for have been given."

Unite, one of the other rail unions involved in the row, announced that its 500 members involved in the dispute will not be going on strike.

Regional officer Brian Harris said: "After days of intense negotiations we’re pleased to announce that Unite is calling off its industrial action at Metronet.

"We have received guarantees from both Metronet and the Mayor that there will be no redundancies or forced transfers whilst Metronet is in administration.

"Furthermore, we have received assurances on our members’ pensions now and going into the future."