The EU Commission's chief negotiator has warned that a deal with the UK on Brexit should be reached by October 2018.

Michel Barnier’s call for the tight schedule, in his first briefing on preparations for Article 50 negotiations, increases pressure on Prime Minister Theresa May to reveal details of her negotiating position.

The EU official, with a reputation for being a tough negotiator, said "third countries" like the UK could not expect to receive the same benefits as full members.

Mr Barnier said: "Should the UK notify the council by the end of March '17 [that it is triggering Article 50], as Prime Minister Theresa May said she would, it is safe to say that negotiation would start a few weeks later and an Article 50 agreement be reached by October '18."

After that date there would need to be four or five months for ratification, he said. His comments mirror those of the European Parliament’s chief negotiator Guy Verhofstadt, who predicted a tense period of 15 months for talks after Article 50 is triggered in March 2017.

Mr Barnier said the EU and UK have "a common interest" in not prolonging uncertainty around Brexit, but stuck doggedly to the EU's negotiating line, that the "single market and its four freedoms are indivisible", highlighting the difficulty Theresa May will have in trying to win access to the trading bloc while imposing tighter immigration controls.

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He said: "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."

Mr Barnier underlined that it is up to the UK tell the EU what it wants from its future relations and for the 27 member states to respond.

The EU negotiator went on to suggest a transitional period could be agreed, but that it is difficult while there is so little known about what the UK requires from its future relationship.

He said: "Until we know what the intentions of a request from the UK are, what they would like and what they are prepared to accept with this new partnership, it’s going to be difficult to talk about a transitional period.

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"The term transitional period only has sense if it prepares the way for a future relationship, and once again, first and foremost, we need to know the dimensions, the content, the new dimension of what this relationship is going to be in order to see how to possibly prepare it."

Conservative MP Andrew Tyrie, Chairman of the Treasury Committee, said: "Mr Barnier’s latest contribution is of just the type calculated to raise the political temperature at a time when he should be lowering it. He should have the economic wellbeing of Europe and its citizens as his overriding objective, not grandstanding to Brussels.

"Both sides in the negotiations can gain together or lose together. So his first priority should be thinking about how to re-establish a close economic relationship between the EU and the UK, after Brexit."

It came as Ms May faced domestic pressure to set out her Brexit plans, even frustrating some of her own MPs with her reluctance to spell out her goals.