Carlos Tavares, the boss of France's PSA Group, has given assurances to the Unite union about UK plants following a potential deal to buy carmaker Vauxhall.
PSA Group, which makes Peugeot and Citroen cars, wants to buy General Motors' Vauxhall and Opel businesses.
Vauxhall employs 4,500 staff at plants at Ellesmere Port and Luton.
Unite union leader Len McCluskey said he was optimistic after meeting Mr Tavares.
"He [Mr Tavares] was very pro-union. He talked in terms of not being here to shut plants. That's not his nature.
"He pointed out that since being chief executive of Peugeot, he hasn't shut a single plant. So I'm going to take him on face value at the moment and hope that I am not disappointed in the future."
Mr Tavares is set to meet business minister Greg Clark to discuss the proposed takeover on Friday.
About 23,000 people work in Vauxhall's retail network and 7,000 in its UK supply chain.
Unions in Germany and the UK have been concerned that a takeover of Opel and Vauxhall could lead to job losses.
Should the PSA deal go ahead, GM would leave the UK and Europe, while PSA Group would become Europe's second-largest carmaker - behind VW - supplanting Renault-Nissan.
The French state has a 14% stake in PSA, as do the Peugeot family and Chinese company Dongfeng Motor.