Lord Mandelson will today come under renewed pressure from the UK's biggest union to shore up the future of 5,000 Vauxhall car and van workers at factories in Ellesmere Port, Cheshire and Luton.

As the Business Secretary prepares to visit Vauxhall's Luton IBC van plant today, Unite urged him to give a "100 per cent pledge" that he would fight for job guarantees from Magna, the Canadian business that is almost certain to be the new owner.

Magna agreed to take control of GM Europe, which owns Vauxhall and Opel factories, in May, as General Motors, the American parent, was preparing to file for Chapter 11 bankruptcy protection in one of the biggest corporate failure in US history.

Magna has proposed paying €700 million (£610 million) for a 55 per cent slice of Opel and has handed over €300 million of emergency funding to keep GM Europe afloat.

But the bid to buy GM Europe is conditional on due diligence of both Opel and Vauxhall.

Lord Mandelson has already met Magna executives in Berlin in recent weeks to try to secure guarantees on Vauxhall's UK factories, but no firm commitments have yet been forthcoming.

Magna's proposal envisages up to 10,000 jobs being lost across GM Europe.

In return for offering to guarantee UK jobs, the Canadian company wants financial support from the UK Government of up to £350 million.

Vauxhall's Luton van plant employs 1,450 staff, provides work for a further 500 in-house contractors and supports another 4,000 jobs across the components and supply chain.

Today, Unite called on the Government to do all it can to protect Vauxhall jobs, not just at the Luton plant, but across Britain.

Rob Weir, a senior Unite shop steward at the Luton plant, said: "This plant is one of the most productive and efficient in Europe. We set the benchmark which other plants must follow and we are a brilliant advert for British manufacturing.

"So what we want to hear from Lord Mandelson today is that the UK Government is 100 per cent behind us and 100 per cent engaged in securing the Luton plant's future."

During his visit, Lord Mandelson will be presented with a model of the Vivaro, a van made at the plant for the past nine years which has won annual Van of the Year awards six times during the period

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