West Midlands Mayor Andy Street has warned hard-line Brexiteers they risk causing "the unintended destruction of thousands of jobs in the automotive industry" in the region.
The Conservative mayor weighed into the battle taking place within the Cabinet over the type of deal the UK should be negotiating with the European Union.
He said that making the wrong decision could lead Tata Motors, the owners of Midlands-based carmaker Jaguar Land Rover, to build vehicles in other parts of Europe instead.
Mr Street said: "The Brexit decisions that the Cabinet take over the next few weeks will tip those boardroom decisions one way or the other, and I want to make sure the investment in the West Midlands car industry continues."
Prime Minister Theresa May is understood to want a "Customs Partnership" with the EU after Brexit. This would retain many of the benefits of membership of the EU Customs Union.
She's backed by Business Secretary Greg Clark, who said the Prime Minister's plan is the best way to protect the car industry.
But the proposal is opposed by Brexiteer members of the Cabinet such as David Davis, the Brexit Secretary.
And Mrs May faced an unprecedented rebellion from Boris Johnson, the Foreign Secretary, who described her idea as "crazy" in a newspaper interview - ignoring the convention that Cabinet Ministers only disagree with each other in private.
Customs arrangements are important to the automotive industry because manufacturers import parts provided by suppliers from across the EU.
Writing in the Times, Mr Street said carmakers would only continue investing in the UK if post-Brexit arrangements "keep those parts moving quickly and predictably".
He warned that Tata Motors was in the process of choosing a location to build electric cars, which will compete with vehicles produced by US firm Tesla.
Mr Street said: "We have the talent here, but will the Government’s Brexit decisions convince the board that they should build their rival to Tesla’s Gigafactory right here in the West Midlands?
"We know other countries in Europe will try to step in if we falter."
He stressed that allowing parts to be imported into the UK without delays is essential to the success of the UK car industry.
"As the Cabinet examines the different systems for administering customs after Brexit, they must ensure that whatever the final arrangements are, they sustain the manufacturing renaissance we have seen in the West Midlands.
"Manufacturers will look to see if the new system can keep those parts moving quickly and predictably. It’s not possible to run world-class lean manufacturing processes if you don’t know when a component will be delivered."
But carmakers would "struggle" if there were delays importing parts into the UK, he warned.
"If the technology doesn’t work for car parts at Dover or Felixstowe, if there are big delays, new checks or burdensome processes to claim back customs paid, costs will go up, and companies will struggle.
"If we get this wrong, we will see the unintended destruction of thousands of jobs in the automotive industry in the West Midlands."
The intervention by the Tory mayor will be welcomed by Cabinet Ministers who support an arrangement with the EU which retains the the benefits of the Customs Union.
His win in last year's mayoral election was seen as a major boost for the Conservatives, and he represents a region which voted overwhelmingly to leave the EU.
Jaguar Land Rover has major plants in Erdington and in Solihull.
