Ford and Ryanair warned on Wednesday of the risks of Brexit including disruption to flights and tariffs on cars which could hurt Britain and damage businesses, on the day the prime minister was launching divorce proceedings from the EU.
U.S. carmaker Ford, Britain's biggest automotive engine-maker, low-cost airline Ryanair and German media group Bertlesmann
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"Any deal must include securing tariff-free trade with the wider Customs Union and not just the EU27, whilst retaining access to the best talent and resources," Ford of Europe president Jim Farley said.
"It also is critical that a transitional period is put in place to ensure that customers are not penalized and to maintain free trade."
Turkey is not part of the EU but is in the EU customs union.
Most international firms which publicly expressed an opinion ahead of last June's referendum backed Britain remaining in the European Union, fearful of extra costs, trade barriers and unpredictable currency swings.
May has said she will take Britain out of the European single market but will seek the best possible access to the European markets and establish better trade ties with other nations.
Since the Brexit vote, some firms have announced major investments in Britain with Facebook
But others are concerned that trading conditions vital to their operations could be lost.
Irish airline Ryanair
Britain will have to renegotiate access to the single aviation market, whereby airlines based in the EU have the right to fly to and from any country in the bloc or even within other member states.
German media conglomerate Bertelsmann
"We have made an impact analysis," Chief Executive Thomas Rabe said.
"In about a year's time we will have to come to a decision, when the impact of the Brexit will become more clear."
(Additional reporting by Conor Humphries in Dublin and Harro Ten Wolde and Jörn Poltz in Berlin; editing by Stephen Addison)