Metronet went into administration after a projected �2bn overspend

Members of the Unite and Rail, Maritime and Transport (RMT) unions will hold 72-hour strikes on 3 and 10 September.

A 48-hour strike by the Transport Salaried Staffs' Association (TSSA) will begin on 4 September.

They are protesting over job losses faced by Metronet staff. Transport for London said all issues raised by the unions were being addressed.

Tube maintenance firm Metronet had planned to invest �17bn over the next 30 years, under the terms of a public-private partnership (PPP) scheme, but went into administration in July amid serious financial problems.

Public sector

The strikes could result in a lack of staff to carry out vital track maintenance and safety checks, said the unions.

The RMT said this would have "a massive cumulative impact on Tube services on those lines maintained by Metronet, but the spill-over effect will disrupt the entire network".

The three unions' 2,500 members on the Underground voted overwhelmingly for strike action, earlier this week.

They (RMT) care nothing for the welfare of the travelling public Roger Evans, London Assembly

RMT general secretary Bob Crow said: "The bottom line is that they will not accept being made to pay for the failure of the PPP and the decision by Metronet's fat-cat shareholders to walk away from the contract...

"Our members are the people who get out there and keep the Tube running seven days a week, and it is they who will deliver the improvement the network must have if it is to be up to the standard required by the 2012 Olympics."

But the chairman of the London Assembly Transport Committee Roger Evans condemned the move by the RMT.

Mr Evans said: "This is typically inflammatory action from the RMT leadership, designed to make a bad situation on the London Underground worse.

"They care nothing for the welfare of the travelling public. Their only concern is promoting their own narrow interests while the rest of London can go hang."

Mayor committed

Meanwhile TSSA general secretary Gerry Doherty said: "Our members are not renowned for their militancy and striking for 48 hours is a big step for them.

"However, they have very real concerns that they will be made to pay for the failure of the PPP in the hands of Metronet and they do not feel that their employer or the administrator are listening to them."

A TfL spokesman said: "Threatening Londoners with two three-day strikes on the Tube when no action is being taken to disadvantage any employees is wrong."