Magna, the new owners of Opel and Vauxhall, will cut up to 1,400 UK Vauxhall jobs, according to figures leaked to the German press.
GM's UK plants include Ellesmere Port on Merseyside and Luton in Bedfordshire, alongside a central UK headquarters in Luton. It currently employs around 4,700 people in the UK.
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That would mean around one in three Vauxhall jobs would go under Magna's rumoured plans, while GM operations in Spain and Belgium will also be hit hard.
Countries outside Germany are set to lose around one in three jobs, according to the leaked figures, while losses at German plants are closer to one in five.
Germany looks set to escape the brunt of job losses, confirming fears that Germany's financial support of Opel in recent months and in the future have put foreign plants at a disadvantage.
Other sources within Magna have put UK job losses at a lower 1,100.
Business Secretary Lord Mandelson has warned that the deal between Magna and the German government, currently fighting a general election campaign, may need to be investigated in case of a 'fix' that guarantees favourable conditions for German plants and workers at the expense of those in Spain, Belgium and the UK.
"The European Commission has also said it will be looking closely at the business plans and the financial contributions by the various governments and will not accept anything that looks like a political fix or any requirement that links aid to job retention for any specific plant or country," said a spokesman for the Department of Business, Innovation and Skills.
Around 11,000 of Opel's 54,000 workers are likely to go under restructuring.
General Motors recently agreed to sell a 55% stake in Opel and Vauxhall to Magna and Sberbank.