A split from Europe means that companies building in the UK would no longer be guaranteed tariff-free access to its biggest export market, unless that is specifically negotiated in the departure terms. Last year, more than 900,000 vehicles made in the UK were sold into Europe, according to the Society of Motor Manufacturers and Traders.
In a letter sent to employees ahead of the referendum, Toyota Motor warned that a UK withdrawal from the EU could result in levies of 10 percent imposed on the cars it builds in Britain, Bloomberg reported. Toyota Europe CEO Johan van Zyl told the Automotive News Europe Congress in June that the company could rethink investments in the UK if Britain voted to leave.
GM said it supports the UK remaining part of the European Economic Area, a broader free-trade bloc, to avoid any potential tariffs on cars and vans it makes in the UK.
Jaguar Land Rover could be forced to drastically improve the fuel economy of its cars if the EU voids a special agreement negotiated by UK officials that loosens the carbon-dioxide targets its heavier cars need to reach by 2021.
"Once the UK is outside the EU, why wouldn't the French and Germans get rid of that amendment? There would be no one left inside the EU benefiting from that," John Leech, head of the UK automotive practice at accounting firm KPMG, told Automotive News last year.
JLR declined to comment on that possibility, saying in a statement that "it would work hard with all parties to ensure that the importance of the British automotive industry is fully understood at every level of the negotiation process." The company sells 20 percent of the cars in makes in the UK in Europe, it said.
The UK's biggest vehicle exporter to Europe, Nissan, didn't comment on the result of the vote, but CEO Carlos Ghosn previously said he favored the UK staying in the EU.
BMW, which owns six British companies including Mini and Rolls-Royce, said it was too soon to know the full effect on its operations.
"While it is clear there will now be a period of uncertainty, there will be no immediate change to our operations in the UK," BMW said in a statement, adding that it "respects the British electorate's decision to leave the EU."
Reuters contributed to this report.