LONDON (Reuters):

Millions of tube passengers face three days of travel chaos after unions said yesterday a strike over job security and pensions will go ahead.

The 72 hours of disruption will begin at 6:00 p.m. today, with repeat action planned for the following week.

The strike comes as many Londoners return towork after their summer holiday.

It follows the collapse of Metronet, responsible for maintenance on nine of the London Underground lines.

Transport for London had hoped that letters of reassurance about jobs, transfers and pensions sent out to union members would be enough to stave off the strikes.

But a spokesman for the Rail, Maritime and Transport Workers union (RMT), which has 2,300 Metronet members, said: "My understanding is that we have not received the cast-iron guarantees that were sought.

"The strike is going ahead on Monday."

He said the impact on travellers would build up over the three days.

Services will still run, but any problems with maintenance of trains, tracks or signalling will not get repaired.

"The network will grind to a halt," the spokesman warned.

"If anything goes wrong, it will stay wrong."

Raised concerns met

The Transport for London said in a statement yesterday that the RMT had received letters from their employers, the administrator and metronet, as well as the Mayor of London, Ken Livingstone, assuring them that all of the concerns raised had been met.

It said it had been informed that two other unions involved in the dispute, the Transport Salaried Staff's Association (TSSA) and Unite, will hold a meeting today.

"We fully expect them to recognise that they have now received all of the assurances that they have requested," it said.

Neither the TSSA nor Unite was immediately available for comment.

Livingstone said in a statement: "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."