EU's Brexit official sets 18-month deadline and tells UK to 'keep calm and negotiate'

Although Article 50 of the EU treaties allows two years to thrash out a deal, Mr Barnier said that talks must end months earlier, to allow time for the European Council, the European Parliament and the UK to ratify the agreement.

If Mrs May sticks to her timetable, this would mean a deal was needed by October 2018, in anticipation of the UK leaving the EU in 2019, he said.

But Downing Street signalled that the UK was not committed to this schedule, stating that it would be "wrong" to put a deadline on the conclusion of negotiations. A spokesman pointedly added: "It is our Article 50 as well, because we are members of the EU. The timetable is clear in that."

Mr Barnier indicated a transitional arrangement - potentially meaning the UK continuing to pay into the Brussels budget in order to secure the best single market access - could be put in place after the formal break, in order to avoid the kind of "cliff edge" shutdown of free trade arrangements which Mrs May mentioned in a recent speech to business leaders.

But he said it would be up to the UK to set out its aims first before the rest of the EU could consider a transitional deal.

A transitional period "only has sense if it prepares the way for a future relationship", said Mr Barnier, in his first briefing on his preparations for the Article 50 negotiations since being appointed European Commission chief negotiator by president Jean-Claude Juncker in July. But he also acknowledged that "you can't do everything in 15 to 18 months of negotiations".

Speaking in Bahrain, Mrs May dismissed speculation over whether she is aiming for a "hard" or "soft" Brexit.

"These terms that have been identified: hard Brexit, soft Brexit, black Brexit, white Brexit, grey Brexit. Actually, I think what we should be looking for is a red, white and blue Brexit," said the British Prime Minister.

"That is the right deal for the United Kingdom. What is going to be the relationship for the UK with the European Union once we have left the European Union. That's what we are about and that's what we will be working on."

Mrs May is facing domestic pressure to set out her Brexit plans, with up to 40 potential Tory rebels thought to be prepared to back a Labour motion on the issue on Wednesday.

The vote - which the PM is likely to miss - comes as the Supreme Court considers a separate legal challenge to force the British Government to seek Parliament's approval before launching the withdrawal talks.

But a Downing Street spokesman said: "The Government's position hasn't changed. We don't intend to reveal our negotiating position ahead of triggering Article 50."

In an apparent sign that he is unwilling to budge from the requirement for freedom of movement in return for full access to the single market, Mr Barnier said the UK will not be allowed to "cherry-pick" which EU rights and obligations it wishes to keep.

"Being a member of the European Union comes with rights and benefits. Third countries can never have the same rights and benefits since they are not subject to the same obligations."

He said he was aiming for an "orderly" Brexit, but said it was "too soon" to discuss the details of how it would happen.

"We are entering uncharted waters," said the French former foreign minister.

"The work will be legally complex, politically sensitive and will have important consequences for our economies and for our people, on both sides of the Channel."

Mr Barnier recalled campaigning in his 20s in the French referendum of 1972 in favour of Britain's entry into what was then the EEC, adding that "I still think today that I made the right choice".

He said he had visited 18 of the 27 remaining EU capitals since July and had established a taskforce of 30 experts who were scouring the EU's rules and regulations to identify issues for the Article 50 talks.

"We are ready," he said. "Keep calm and negotiate."

Despite earlier suggestions that he would insist on French being the language of the Article 50 talks, Mr Barnier spoke in both English and French at the Brussels press conference.

Asked whether Mr Barnier's use of the wartime slogan was provocative, a Downing Street spokesman said the UK would conduct negotiations "in a spirit of goodwill and we will do it calmly".

The spokesman said that Article 50 set out a two-year process for withdrawal from the EU.

"We have been clear that we are not seeking to extend that process," he said. "In terms of how long actual negotiations take, clearly that's a matter that will resolve itself as a result of the negotiations. It would be wrong for me to put a timetable on that.

"The European negotiating team's approach to the negotiations between the UK and the other 27 member states is clearly a matter for them. We have been clear on our timetable and we are not moving away from that."

A spokesman for the Leave.EU campaign said that Mr Barnier's warning that there would be no "cherry-picking" confirmed its position that the UK must quit the single market to end free movement.

"It's time for Theresa May to get off the fence and confirm Britain will be leaving the single market," said the spokesman.

"If we do not leave the single market, we have not left the European Union and voters won't forget a betrayal of that magnitude at the next general election."