DONALD Tusk has told Theresa May it is not too late for the UK to change its mind on leaving the European Union as he demanded more clarity from the Prime Minister over her plans for Brexit.

Addressing MEPs in Strasbourg, the European Council president said on Tuesday: "If the UK Government sticks to its decision to leave, Brexit will become a reality - with all its negative consequences - in March next year unless there is a change of heart among our British friends.

"Wasn't it David Davis himself who said 'if a democracy cannot change its mind, it ceases to be a democracy'?"

The December European Council had agreed that "sufficient progress" had been made on the first phase of Brexit talks, allowing negotiations to move on to consider transition and a future deal.

Tusk said: "As regards our future relations, what we need today is more clarity on the UK's vision.

"Once we have that, the leaders will meet and decide on the way the EU sees its future relationship with the UK as a third country."

He insisted that the EU had not had a "change of heart" over Brexit, telling the British: "Our hearts are still open to you."

Calling for continued unity among the remaining 27 members of the EU, he said: "The hardest work is still ahead of us and time is limited.

"We must maintain the unity of the EU27 in every scenario, and personally I have no doubt that we will."

European Commission president Jean-Claude Juncker told the parliament in Strasbourg that he hoped Tusk's message "will be heard clearly in London".

Senior German MEP Manfred Weber, a key ally of Chancellor Angela Merkel, mocked May's plans for a return to blue passports after Brexit.

Weber, leader of the European People's Party group, said: "The whole story is a scam, EU law does not say anything about passport colours."

He added: "The blue passports are the first and the only real thing that the British Government have achieved in more than one-and-a-half years of negotiations."

He also warned a transitional period could not be taken for granted and "the cliff edge is far from being avoided".

The European Parliament's Brexit co-ordinator Guy Verhofstadt said the situation over passports was a "whole hilarious thing".

"That was certainly not a reason to organise a referendum," he said.

Verhofstadt said the negotiations were now coming to the "most difficult part".

The withdrawal agreement must have "serious guarantees" on citizens' rights because "that is still a problem".

The bureaucratic burden should be on the Home Office, not the EU citizen, he said.

A transition deal could not be "cherry picking".

He said the latest version of the EU's negotiating directives made clear that "all the policies of the EU" would apply during a transition period but "Britain will not be represented any more in the institutions" in Brussels.

Verhofstadt said the rights of citizens - including on free movement - should continue to apply until the transition period ends.

"It is very important that in these negotiating directives... the new system for EU citizens living in Britain is only coming into place after the transition," he said.

There was "no question" of trying to "make it difficult for EU citizens to obtain their permit to reside in Britain".

"That can only be applicable after transition," he said.

Verhofstadt said MEPs would reject any suggestion that "you can have a better position, a better status outside the EU than inside".

"Inside the union, membership of the union, will always be the best solution for every European country and European state," he said.

Scottish Labour MP Ian Murray, a supporter of the Open Britain campaign for close ties with the EU, said: "Until we've actually left, Brexit is a reversible process, that much is clear. If people decide that Brexit isn't the right path for the country, they have the right to change their minds.

"The point has been made by legal experts, EU leaders and even the architect of Article 50, Lord Kerr. The Government should now be upfront with the public and publish its own legal advice on the matter."

Meanwhile, a leading health expert has said it is "ridiculous" for Foreign Secretary Boris Johnson to have stated that the Brexit campaign's claim that leaving the EU would result in an extra £350 million a week for the NHS was an underestimate.

Speaking at a press conference in London about reducing health inequalities, Professor Sir Michael Marmot was asked what he made of Johnson's claim.

"I think when the history of the 21st century is written, Boris Johnson will go down as one of the best guests on Have I Got News For You.

"It's a ridiculous thing to say. It's been pointed out by the statistics authority, the national statistics authority that that is totally misleading".

Sir Michael added: "He does say 'gross'. Gross is a good word to use actually... but that's not the relevant bit."

The academic said the claim has been shown to be "a lie", adding: "It's an untruth. It's misleading. I'm not sure what other word to use."

Meanwhile, MPs have been told there is a “strong case” for a longer post-Brexit transition period to mitigate against the potential loss of EU students and staff at Scottish universities.

The Scottish Affairs Committee heard the number of EU nationals coming to Scotland to study could drop significantly if tuition fees are introduced, and some institutions and subject areas would be seriously affected by the decline.

The UK Government has proposed a two-year implementation period after Britain leaves the EU in 2019 but the European Commission’s chief negotiator Michel Barnier has said any transition period should finish at the end of 2020. European law means students from other EU countries are able to access free tuition in Scotland in the same way as Scottish students.

However, Professor Andrea Nolan, convener of Universities Scotland, said she expected this requirement to end after Brexit, adding that Edinburgh Napier University, where she is principal, would introduce fees.