London – Britain will face "unavoidable barriers to trade" with the European Union if it leaves the EU customs union, the bloc's chief Brexit negotiator has warned.

Post-Brexit barriers to trade in goods and services were inevitable "without a customs union and outside the single market," Barnier told reporters following talks in London with Prime Minister Theresa May and Brexit Secretary David Davis.

Time to choose

Barnier said the EU planned to produce its guidelines for trade talks in March, still leaving Britain time to clarify what it wants from the negotiations.

He added that Britain would need to "play by the same rules" of the single market and customs union during an expected transitional period from the date of Brexit in March 2019 to the end of 2020. "The time has come [for Britain] to make a choice," Barnier said.

Davis told reporters that his government had been "perfectly clear" that it wants a comprehensive free trade agreement and a customs agreement, which will allow the continuation of trade to be as frictionless as possible after Brexit.

Barnier's visit to London precedes three days of talks in Brussels in the latest round of formal Brexit negotiations.

No change of position

Earlier, May had dismissed speculation that she could switch her Brexit negotiating position to keep Britain in the customs union after it leaves the bloc.

"It is not our policy to be in a customs union," May's office, Downing Street, said in a statement following claims by pro-Brexit lawmakers in her Conservative party that the government could change its position.

Tom Brake, the opposition Liberal Democrats' Brexit spokesman, said on Twitter that May's statement "confirms the crazed, extreme Brexiteers are now in control of [the] government."

Irish border checks

David Cullinane, Brexit spokesman for Irish Republican party Sinn Fein, said leaving the customs union would lead to checks at the Irish border and jeopardize the 1998 Good Friday Agreement, which underpins the peace process and power-sharing in Northern Ireland.

"There will be a border if the hard Brexiteers get their way and Britain leaves the customs union," Cullinane said. "This is why we need special status [in the EU] for the north."

But Sammy Wilson, a lawmaker in the British parliament from Northern Ireland's rival Democratic Unionist Party, welcomed May's "clarification" of her position.

"Withdrawal issues"

"Last week, I made clear that the DUP expected the PM to deliver on the [Brexit] referendum result and free us from 'the protectionist policies of the customs union'," Wilson wrote on Twitter.

Wilson is one of the 10 DUP lawmakers propping up May's minority government by supporting her on key votes in the British parliament, after she lost her majority in a disastrous snap election in June.

Davis said earlier that this week's negotiations in Brussels would cover "withdrawal issues," the post-Brexit Irish border and the transitional arrangements after March 2019.