The Prime Minister signed a deal with German firm Siemens to build carriages for the £6billion upgrade on the Thameslink rail route, forcing Derby-based Bombardier - which lost out - to lay off thousands of workers.

Mr Cameron and his Transport Secretary Philip Hammond insisted their hands were tied by the tendering process drawn up by the last Labour government.

But the Department for Transport's "invitation to tender" (ITT) for the Thameslink Rolling Stock Procurement Programme (TRSP) proved that is not the case.

The document reads: "The issue of this ITT in no way commits the Secretary of State to award the TRSP to any person or party.

"The Secretary of State reserves the right to terminate the competition, to award the TRSP without prior notice, to change the basis, the procedures and the timescales set out or referred to in this document or to reject any or all Proposals and to terminate discussions with any or all Bidders at any time."

That contradicts what Mr Cameron said at Prime Minister's Questions last week. "In this case the procurement process was designed and initiated by the previous government," he said.

"We were bound by the criteria they set."

Mr Hammond, another multi-millionaire Tory, claimed: "I did not have the flexibility to award the contract to Bombardier."

But Labour and the unions said the leaked document proved ministers can intervene. Former Treasury Minister Geoffrey Robinson said: "This document proves ministers retained the power to put the national interest first. Highly-skilled jobs will be sacrificed, and a vital industry lost, unless the decision is reversed."

And Unite leader Len McCluskey said: "Mr Hammond and Vince Cable, stop bumbling about. You are the Government of this country and we expect you to bend over backwards to get behind Bombardier and its workforce."

The DfT last night desperately tried to play down the document. A spokesman said: "The purpose of the Important Notice in the 2008 TRSP ITT is to allow minor changes.

"However any such changes must be non-discriminatory and must not contravene EU procurement directives and public procurement law."