Concerns have been raised about Vauxhall's 35,000-strong UK workforce as General Motors (GM) considers the sale of the loss-making arm to Peugeot-owner PSA Group.
Unite's general secretary, Len McCluskey, said it was his priority to speak to GM about what impact the proposed deal would have on the employees.
PSA Group, which also owns Citroen and DS, is locked in talks over a "potential acquisition" of Vauxhall and Opel, as it explores "strategic initiatives" aimed at boosting profits and cutting costs.
A deal would transform PSA Group into Europe's second-largest carmaker with a 16% share of the European market and see GM exit UK and Europe.
Vauxhall employs 4,500 staff at plants in Ellesmere Port and Luton, with a further 300 people working in a customer contact centre and 120 at its OnStar headquarters.
Around 23,000 people also work in Vauxhall's retail network, while 7,000 jobs rely on the firm's UK supply chain.
My priority now is to speak to General Motors to seek immediate assurances for the UK plants and this loyal workforce.
ITV News Business Editor Joel Hills, tweeted a letter sent by GM's Chairman and Chief Executive Officer, Mary Barra, to Vauxhall workers confirming that talks are taking place.
GM said last year that it had to raise UK car prices by 2.5% after the EU referendum result caused the British car industry to hit a "speed bump".
The Detroit-based firm added that its European arm was on course to break even before the plunge in the value of the pound following the Brexit vote.