The decision came as London Underground's governing body, the Board of London Transport, voted unanimously to proceed with modernisation plans for the Tube.

This means that, for the first time, secure and sustained investment to improve and maintain the infrastructure of the world's oldest underground railway is now locked in for a generation.

London Transport's decision follows an extensive period of consultation with the Mayor's Transport for London, resulting in changes and improvements sought by TfL being taken on board in this process.

Contracts have now been signed between London Underground and the two consortia - Metronet and Tube Lines Group - responsible for delivering improvements under the PPP.

Over the next few weeks' clearance will be obtained from the competition authorities, private finance will be raised and the successful bidders are set to start work fixing the Tube in the summer.

In parallel, London Underground will continue to work closely with the Health and Safety Executive to reach acceptance of its Safety Case for PPP - a critical milestone that must be achieved before the work can begin.

London Underground's Managing Director, Paul Godier said: "The Tube has today moved one vital step closer to the essential, long term funding it so desperately needs.

"It has been a long hard slog to get where we are today. We are all working flat out to make sure the final pieces of the jigsaw are in place so that work can start - without any further delay - to deliver a world class Tube for a world class city."