The Government was "betraying" British industry by awarding a major train-building contract to a German firm, the Labour Party conference was told.
The decision to award the work to Siemens instead of Britain's last remaining trainmaker Bombardier could cost 10,000 jobs, the gathering in Liverpool heard.
Labour activist Greg East said the 1,400 jobs had already been lost at the Derby-based firm but thousands more posts could be axed in the supply chain.
Mr East, from Derby North Labour Party, said: "This industry has served Britain loyally and now the Tory-led Government is betraying it.
"Train design, manufacture and assembly have existed in Derby since 1840 and the city has a tradition steeped in rolling stock manufacture.
"The Government decision not to award the preferred status to Bombardier will have a profound effect on the city and the country as a whole."
Since the announcement that Bombardier missed out on the work to build trains for the Thameslink route, thousands of people have publicly lent their support to the rail manufacturers.
Mr East said: "The decision has led to the loss of 1,400 highly-trained, highly-motivated and highly-skilled employees.
"However, the real problem is the loss of skills and engineers in my city.
"This will not only lead to the loss of skills to the region, it will have an impact of huge proportions on the supply chain in Derby and Derbyshire.
"It's estimated that 10,000 skilled workers will lose their jobs. Can the economy really afford this at a time of great uncertainty and fear about a renewed global recession?"
The conference passed a motion condemning the "disastrous" decision which will "effectively end the UK's train manufacturing capability".