Railtrack, which owns Britain's railway tracks and major stations, has emerged as the key financial backer that could save the project.

Imperial College Management School's Stephan Shemanski questions the need for the rail link

The rail link is likely to be built in stages with Railtrack expected to buy the first phase from London to Ebbsfleet from London & Continental Railways (LCR), the original project consortium which fell into financial trouble in January.

John Prescott, the Deputy Prime Minister, is reported to be ready to unveil the rescue package in a Commons statement.

However a spokeswoman for the Department of the Environment, Transport and the Regions said: "The Deputy Prime Minister said he wanted to make an announcement as soon as possible after the (Whitsun) recess, But I am not in a position to confirm any particular date at this stage."

John Prescott is expected to announce the rescue package tomorrow

The government is understood to have agreed to contribute an extra £700m over the project's 10-year life.

That comes on top of the £1.8bn it has already committed but is less than the £1.2bn LCR had originally called for in January when the full extent of its problems came to light.

The Eurostar train service will be operated by a consortium led by British Airways and National Express or Richard Branson's Virgin Group.