"I have in the last 24 hours spoken again to the senior executives of General Motors in Europe and again I have got their commitment reaffirmed to me - as I did previously, in the previous week with Magna - that their commitment to Vauxhall production continuing in the United Kingdom is firm," he told the BBC's Politics Show.
"But we've got now to pin down specific plans and specific implications for jobs."
He added that Magna had to date been vague both on its plans for the business and the implications for jobs.
But the rescue deal with Magna looks likely to spare GM's European arm from bankruptcy.
GM is expected to file for bankruptcy protection early on Monday in what would rank as one of the biggest bankruptcies in US history.
Lord Mandelson said that Magna's proposal to take over Vauxhall and Opel could threaten a key contract between Renault Nissan and Vauxhall's van making plant in Luton. Renault builds its Trafic van in Luton and the contract is set to run until 2012.
Anxiety among Luton firms grows
He told the BBC that Renault Nissan has a clause in its contract that states that it can renegotiate or even end the agreement with Luton, should the ownership of the plant change - as would be the case if Magna completes its deal to take over GM Europe.
"These are among the many details and specifics of Magna's plans that we have to discuss with them and tie down," Lord Mandelson said.
"If they want British government to help underwrite this new company going forward, they are going to have to demonstrate what's in this new arrangement for Britain, for British production and British workers."
Derek Simpson, general secretary of the Unite union, said on Saturday he feared German plants will be saved rather than UK factories because the German government had offered financing.
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