A group of major European companies has warned the Prime Minister they may cut investment without more clarity over the terms of Britain's EU exit.
Business leaders, including from BP, BMW, Nestle, and Vodafone, told Theresa May that "time is running out".
In a statement after the Downing Street meeting, they said that a trade deal with the EU must be "frictionless as with a customs union".
Downing Street said that the meeting had been "open and productive".
The industry leaders warned that "uncertainty causes less investment."
The group, known as the European Round Table of Industrialists (ERT), includes BP chairman Carl-Henric Svanberg and his Nestle counterpart Paul Bulcke.
Others at the meeting were the chief executives of Vodafone, Vittorio Colao, and Royal Mail, Moya Greene.
Senior figures from firms including BMW, Phillips, E.On and Ferrovial also attended the meeting with Mrs May and Brexit secretary David Davis.
The ERT represents Europe's 50 largest companies, with combined revenues of 2.25 trillion euros (£2tn) and millions of employees.
In its statement, the ERT said: "The uninterrupted flow of goods is essential to both the EU and UK economies. This must be frictionless as with a customs union.
"We need clarity and certainty, because time is running out. Uncertainty causes less investment."
A Downing Street spokeswoman said that the PM told the business leaders that work was under way on two customs models, "and underlined the importance of ensuring that our future trading arrangements with the EU are as frictionless as possible".
She restated a commitment to avoid a hard border between Northern Ireland and Ireland, and allowing the UK to pursue an independent trade policy.
"The PM recognised the necessity of providing certainty for businesses, pointing to the agreement of an implementation period at the European Council in March to provide time to allow businesses to prepare for the new arrangements," the spokeswoman said.