THERESA May will be asked to guarantee good conduct by the UK as a condition for getting a new Brexit extension, Donald Tusk has suggested.

The European Council President made clear in a letter to EU leaders last night that he was not in favour of May’s request for another short delay until the end of June, warning it was likely to be followed by a further request, prolonging uncertainty for citizens and businesses.

Instead he urged leaders taking part in today’s special summit in Brussels to consider a longer “flexible” delay. But Tusk warned such an offer should only be made if May gave a commitment not to disrupt the workings of the European Union as the UK continued with the process of leaving the bloc.

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He said in his letter: “One possibility would be a flexible extension, which would last only as long as necessary and no longer than one year, as beyond that date we will need to decide unanimously on some key European projects ... Importantly, a long extension would provide more certainty and predictability by removing the threat of constantly shifting cliff-edge dates.

“Furthermore, in the event of a continued stalemate, such a longer extension would allow the UK to rethink its Brexit strategy.”

Following next month’s European Parliament elections, the EU starts its new session in July, with important decisions over the budget and the election of key figures to be decided.

Tusk raised the prospect that May would have to give a commitment that UK politicians would not disrupt the workings of the EU during any new extension period.

“Some of you have raised concerns that the UK’s continued presence as a departing EU country would pose risks for the functioning of the EU27 at a time of key decisions on its future,” he wrote. “To address them we would need to agree on a number of conditions: no re-opening of the withdrawal agreement; no start of the negotiations on the future, except for the political declaration; and the UK would have to maintain its sincere co-operation during this crucial period, in a manner that reflects its situation as a departing member state.”

Tusk also suggested the possibility of a UK change of heart about leaving the bloc, pointing out the possibility of revoking Article 50.

He said: “We should remember, however, that the United Kingdom will remain a member state with full rights and obligations. And, in any event, the UK can revoke Article 50 at any time, as stated by the European Court of Justice."

May is heading to Brussels to ask for an extension until June 30. But she is likely to have to accept a further delay to Brexit until the end of the year, meaning she faces the prospect of more ministerial resignations. Any extension has to be agreed by all EU27 countries.

The Prime Minister lost two members of her government last week after she agreed to sit down with Jeremy Corbyn for talks to break the impasse following a previous extension to the Article 50 process granted by European leaders.

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May arrived in Paris last night, in a bid to urge the French President Emmanuel Macron to back her case for a delay to Brexit.

The National:

Earlier, in what has been described as a “Brexit begging tour” she travelled to Berlin where she met German Chancellor Angela Merkel, above, for a “working lunch” over which they agreed on the importance of an “orderly withdrawal” from the EU, Downing Street said.

In recent days Macron has warned that an extension to the Article 50 withdrawal process is not guaranteed.

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The visits came as the EU’s chief Brexit negotiator, Michel Barnier, said Brussels could amend the political declaration on future relations with the UK “within a few hours or days” to incorporate the customs union arrangement being discussed in cross-party talks between the government and Labour.

But there were signs of resistance in May’s Cabinet against compromising with Labour, with International Trade Secretary Liam Fox warning a customs union would leave the UK “stuck in the worst of both worlds”.

The leader of the Commons, Andrea Leadsom, urged May to ask Merkel to reopen the withdrawal agreement forged with the EU last November – something the EU has repeatedly said it will not do.

Leadsom told ITV News: “The Prime Minister is off to see Angela Merkel today and it would be

fantastic if Angela Merkel would try to support a proper UK Brexit by agreeing to reopen the withdrawal agreement.

“There have been rumours over the weekend that some senior members of the German government would be willing to do that in order to get Theresa May’s deal over the line.”

But Barnier told a press conference in Luxembourg: “The withdrawal agreement is not going to be reopened, is not up for negotiation again. That continues to be the case.”