May says Britain and European Union to start talks on future ties immediately

On the second day of a summit in Brussels, leaders will agree that "sufficient progress" has been made after a deal on citizens rights, the Irish border and Britain's outstanding payments, giving negotiators a mandate to move onto the main phase of talks.

However, while talks will begin in January on a post-Brexit transition period of about two years, actual negotiations on future trade ties would not start until March.

Asked at a Brussels press conference whether Mrs May's goal of concluding negotiations by March 2019 was achievable, Mr Tusk said: "It is still realistic and of course dramatically hard".

She said Britain would be "beginning the talks about our future relationship straight away", adding: "There is still more to do but we are well on the road to delivering a Brexit that will make Britain strong, prosperous and secure". "We have to make sure [a country that leaves] doesn't stay in a transition stage for ever - it wouldn't work for either the United Kingdom, nor the EU", Irish prime minister Leo Varadkar told press after the meeting.

As May left to return to London - she will not join the other 27 leaders for further discussions on Brexit and the euro zone - she said she was eager to move on, once her peers give the formal green light to trade talks on Friday.

The EU has published its guidelines for phase two of the negotiations, with discussions on future economic co-operation not likely to begin until March.

The UK will also need to continue to respect the rulings of the the EU's top court, the European Court of Justice during the transition period, an anathema for hardcore Brexit supporters.

After months of hard talks, May and Juncker agreed a deal on the key divorce issues of Britain's exit bill, the future of the Irish border and expatriate rights.

The document "calls on the United Kingdom to provide further clarity on its position on the framework for the future relationship".

EU leaders said it was for Britain to put forward concrete proposals for the kind of trade deal they want, with Dutch PM Mark Rutte saying Mrs May had so far been "holding her cards close to her heart" but it was now time for her and the UK Government to make clear exactly what relationship they are seeking with the single market and customs union.

Meanwhile, Mrs May is facing a further challenge to her authority next week when MPs vote on a Government amendment to enshrine the Brexit date of March 29 2019 in law.

May replied on Twitter, thanking Tusk and European Commission President Jean-Claude Juncker and hailing it as a major advance in the negotiations.

A British government official said the prime minister was approaching the next phase, which will discuss a transition period as well as the terms of the future trading relationship, "with ambition and creativity".

Praising Mrs May as a "tough, smart and polite" negotiator, he said he was "entirely convinced" that the final agreement reached would be approved by the United Kingdom and European Parliaments.