Britain's largest union is seeking a guarantee for Vauxhall's future after its owner, France's PSA Group, agreed to merge with Fiat Chrysler.
The merger created the world's fourth-largest car company but raised concerns about continued investment in Vauxhall.
United Nations civilian officer Des Quinn said he wanted an emergency meeting to "mitigate uncertainty."
Vauxhall has 3,000 employees in the UK, most of which work at the Ellesmere Port and Luton factories.
Fiat Chrysler and PSA (which also owns Peugeot, Citroen and Opel) confirmed a $ 50 billion (£ 30.8 billion) merger transaction on Wednesday.
The two companies said the goal of the deal is to save $ 4 billion in annual costs through shared purchase agreements and integrated technology.
The two companies said they have no plans to close the plant. But when the two companies were in talks earlier this year, speculation was made that Vauxhall was vulnerable to any restructuring.
Mr Quinn said, "Unity is seeking the highest level of assurance to ensure the long-term future of all PSA UK factories and their highly skilled world-class workforce.
"It is vital that such meetings be held as soon as possible to alleviate employees' natural and reasonable concerns in the face of change and uncertainty."
He said the popularity of Vauxhall and Fiat cars in the UK meant that there was an industrial logic to sustain the British presence.
PSA boss Carlos Tavares will become the CEO of the new company and will have a place on the board.
The deal is Fiat Chrysler's second attempt at a merger this year.
The automaker initially offered to merge with French rival Renault for £ 29 billion, but withdrew from the transaction in June with the intervention of the French government, which owns a 15% stake in Renault. The deal will create the world's third largest automaker.
The merger of Fiat Chrysler and PSA in France is expected to be completed within the next 12 to 15 months.
Before the merger is completed, China Dongfeng Motor Group will reduce its 12.2% stake in French automakers by selling 30.7 million shares to PSA.
In November last year, U.S. rival General Motors sued Fiat Chrysler, claiming that the company had bribed union officials for years to gain GM's millions of dollars in losses.
Fiat Chrysler has denied the allegations, saying GM tried to undermine its merger plan. Fiat Chrysler's boss Mike Manley said on Wednesday that the lawsuit did not affect the terms of the merger. Undisclosed China funded Michigan research institute fined $ 5.5 million