Lord Mandelson, the Business Secretary, has secured pledges from each of the three main potential bidders for Vauxhall to safeguard the vast majority of the carmaker’s 5,000 British workers, it emerged last night.

He extracted the promises from Fiat, the Italian carmaker, Magna International, the Canadian car components maker, and RHJ, the Belgian private equity firm, at recent meetings in Britain and abroad. It is understood that Lord Mandelson has been assured that, although there will be some limited rationalisation to the UK workforce, the future of the car manufacturer in Britain is guaranteed.

Although the Business Secretary has conceded to all three bidders that GM Europe, the holding company that owns Vauxhall in the UK and Opel in Germany, suffers from excess costs and is selling vehicles into a depressed market, he has made it clear that the amount of money Britain will commit to them will depend on the level of job guarantees in the medium term and the long term. “Each of the bids envisages government support but we are some way off from a discussion about governments’ role in any commercial outcome to these discussions,” he said.

The assurances represent a significant victory for Lord Mandelson, who has been desperate to make sure that the German Government does not give in to election-year pressure with a pledge to protect domestic jobs at the expense of those in the UK. Berlin’s view is crucial because it is being asked to stump up billions of euros in loan guarantees as part of any deal.

Magna calculates job losses of 9,000 across Europe – 2,500 in Germany. But before a Chancellery meeting last night, Magna hinted that it could shift the production of the Opel Astra from Antwerp, Belgium, to Bochum, Germany. Under that scenario, job losses in Germany would total only 300.

Fiat has promised to cut fewer than 10,000 jobs at GM Europe. That could entail closing an engine factory in Kaiserslautern, Germany. Unions fear that Fiat’s product overlaps with Opel and Vauxhall will mean redundancies could be much higher than promised.

Although Magna and Fiat are talking of finding “another mission” for the Vauxhall Vivaro van plant in Luton, it is understood that the assurances made to Lord Mandelson broadly safeguard the bulk of jobs there in the short and, possibly, medium term.

Even though the three bidders submitted individual business proposals, which are being scrutinised by Lord Mandelson, it is known that he favours a consortium of the bidders. He has not met China’s Beijing Automotive Industry Corporation, which has also sumbitted an offer.

Separately, the German Finance Minister Peer Steinbrück said the country could provide any new investor in Opel bridge financing of €1.5 billion (£1.3 billion) to help to buy time, should General Motors, the US parent, file for bankruptcy protection at the weekend. To help with the disposal of its European plants, and prevent them being sucked into bankruptcy proceedings, GM said it would combine the assets of Opel and Vauxhall. The European Commission has agreed to a meeting of ministers from all EU countries with an interest in the Opel and Vauxhall units amid growing frustration at Germany’s role in deciding which bidder will acquire them.

GM is not expected to file for bankruptcy this week despite the failure yesterday of a $27.2 billion (£17 billion) debt-for-equity swap vital to keeping the company out of Chapter 11. Sources said that the GM board will announce its decision on the carmaker’s future on or close to the June 1 deadline set by President Obama for GM to cut its debt to win extra government funding.

Shares in GM fell by 29 cents, or nearly 20.14 per cent, to $1.15 at the close of trading in New York as investors prepared for their holdings to be wiped out.

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