Britain's last remaining train-making company is expected to announce around 2,000 job losses at its UK plant.
The bad news from Bombardier follows its losing out to Germany on a lucrative contract to build 1,200 new rail carriages at its plant in Derby as part of the £6 billion Thameslink rail upgrade.
Tuesday's announcement, if as bad as feared, will cut the workforce by two thirds at Derby, whose railway history dates back to 1840.
A consortium led by Bombardier had been competing with a consortium led by Siemens of Germany for a contract for 1,200 new carriages as part of a £5 million upgrade of the Thameslink rail route. Last month the Government announced that Siemens would be the preferred bidder for the contract for Thameslink, which runs from Bedford to Brighton.
Announcing the winner, Rail Minister Theresa Villiers said the bid by Siemens, who will build the new carriages in Germany, represented the "best value for money for taxpayers" and stressed that the contract would create up to 2,000 new UK jobs.
But it was vital for Bombardier's future that it won the Thameslink contract as most of its other contracts are due to finish in September this year. Bombardier does have a Tube train contract that will go on until 2014 but this will only support a few hundred of the jobs at Derby.
Bob Crow, general secretary of the RMT union, said, "No-one should underestimate the devastating consequences of the Government sending the Thameslink fleet contract to Germany. Thousands of skilled workers are now under threat of ending up on the dole as a result of this grievous hammer blow to train-building in the UK."
Unite union co-ordinating officer Mark Young said: "The country that invented the railways is at risk of losing its ability to manufacture trains unless the Government urgently rethinks its decision to award preferred bidder status to Siemens instead of Bombardier.
"The Government can reverse this decision and get UK train manufacturing back on track. It is simply unsustainable for the Government to claim to support UK manufacturing with one hand and then with the other hand take decisions which potentially wipe out an entire manufacturing sector."