LONDON - The European Union's chief negotiator Michel Barnier has warned that the United Kingdom faced "unavoidable" trade barriers after Brexit if it also quit the bloc's single market and customs union, urging UK Prime Minister Theresa May to make her choice.

After talks with May and Brexit minister David Davis in London, Barnier repeated that the EU needed "clarity" on what they wanted from the future relationship.

"Without customs union and outside the single market, barriers to trade in goods and services are unavoidable," he said.

"Time has come to make a choice."

During a visit later to the European Banking Authority, which will relocate to Paris after Brexit, Barnier repeated his warning that financial services would be impacted.

"When the UK leaves the single market, the financial passport is gone," he tweeted, referring to the rights that allow London banks to offer services across the bloc.

May said Britain will leave the EU's single market and its customs union after Brexit, to ensure it takes back control of its laws and is free to strike new trade deals.

But critics warn her move will hit the British economy, and the subject is the cause of intense debate between pro-European and pro-Brexit factions in parliament.

As both sides up the pressure on the prime minister, senior ministers were scheduled to hold two days of talks on Wednesday and Thursday to try to thrash out a plan.

Brexit negotiations were scheduled to resume on Tuesday on a two-year transition period after Britain's withdrawal in March 2019, followed by the start of trade talks in April.

Barnier said he expected an official position from London on the future relationship "in the next few weeks".

'Play by same rules'

London and Brussels struck a preliminary deal on key separation issues in December, and this week begin technical talks on a transition period to smooth Britain's withdrawal.

Both sides agree that relations during that period should continue largely as they are now, with Britain maintaining trading ties in return for keeping up its budget contributions, but without having any say in making policy.

However, May risked upsetting Brussels last week by questioning the rights of EU citizens who arrive in Britain during that time.

"The conditions are clear, very clear. Everyone has to play by the same rules during this transition," Barnier said.

Having previously set a deadline for October to ratify any deal before Brexit day, he warned there was "not a moment to lose".

Afp - Xinhua