EU leaders have warned Theresa May that the second phase of Brexit talks will be harder and “more challenging” than the first, as the spotlight turns to the difficulties Britain’s negotiators will face in the coming months.

The EU27’s new red lines, released at the end of the Prime Minister’s most successful Brussels summit since taking office, show that the EU will make a series of demands already on their way to enraging hardline Brexiteers in Ms May’s Cabinet and party.

The new European Council guidelines to their negotiators show there will be no full trade talks between the two sides until next March, and that UK will be made to implement all new EU rules created by the EU27 during the transition period, without any say in drawing them up.

EU negotiators are also preparing to demand that Britain follow EU customs rules while in its two-year transition period, a move that would bar the UK from inking the third-party trade deals beloved of Cabinet ministers like Liam Fox.

The demands have already angered Tory Eurosceptic hardliners, who see negotiating trade deals around the world and freedom from Brussels regulations as key red lines in their vision for a post-EU Britain.

“The second phase will be more demanding, more challenging than the first phase,” European Council President Donald Tusk told reporters in Brussels at a press conference following EU leaders’ meeting.

German chancellor Angela Merkel echoed his comments. Speaking at a separate joint press conference with French President Emmanuel Macron at the summit, she warned that the UK needed to present more detail before talks could begin in earnest.

“The most difficult phase is ahead of us. Britain has to tell us what they want,” she said.

Speaking alongside Ms Merkel, Mr Macron said: “We’ve been able to maintain the unity of the 27, the integrity of the single market and compliance with our common rules. We’ll ensure that we maintain these same principles in the second phase.”

Ireland’s PM Leo Varadkar said talks had reached a “staging point” but that all issues were far from resolved.

“We’re only at the end of phase one,” he said, adding: “We’re going to have to stay very vigilant. There certainly isn’t any room for complacency.”

Commission President Jean-Claude Juncker also confirmed that the “real” negotiations in the second phase would only start in March. He said before that point, chief negotiator Michel Barnier would run a small number of seminars to scope out the EU27 countries’ views on a trade deal.

If that’s what it’s going to be, then we can go to no deal and we’ll have our £39bn back Andrew Bridgen, Conservative MP

“I think that the real negotiations on the second phase will start in March next year,” he told reporters in Brussels.

But sounding a note of caution, he added: “I cannot say when these negotiations will be concluded, but hope I don’t have to has an as early morning meeting with the Prime Minister than the one I had last week.”

Reacting to the new red lines, Conservative MP Jacob Rees-Mogg, a leading Brexiteer, told Sky News: “If the acquis, the ECJ and free movement remain we would not be in an implementation period but would still be de facto in the EU.”

Macron and Merkel held a joint press conference (Getty Images)

He added: “I assume that Her Majesty’s Government will make its own proposals and not roll over in the way it did at the beginning of the process.”

After hearing of the EU’s stance for the transition, Conservative backbencher Andrew Bridgen told The Independent: “It doesn’t sound like much of a transition to me.

“If that’s what it’s going to be, then we can go to no deal and we’ll have our £39bn back.”

Eurosceptic Jacob Rees-Mogg criticised the proposals (Getty)

Foreign Secretary Boris Johnson has also laid out in certain terms that he would not accept the implementation of new EU laws during the transition period.

The MP for Northwest Leicestershire said that key to any transition period must be the ability for the UK to negotiate trade deals, so that they are ready to sign the moment the transition period ends, and that Britain does not accept new regulation.

But he added that if the terms of the period allowed that to happen, he would be willing to accept continued jurisdiction of the European Court as a compromise.

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Julian Jessop, chief economist at the right-wing think-tank Institute for Economic Affairs’ Brexit Unit, said the plans risk the UK “being boxed in permanently to a ‘soft Brexit’ where it retains the obligations of EU membership with no control over the rules”.

He added such a state of affairs would give “none of the benefits of leaving”.