Britain could one day rejoin the European Union after Brexit, Brussels' most senior official has said.

European Commission president Jean-Claude Juncker said he hopes "the day will come when the British re-enter the boat".

At a press conference in Brussels after a meeting of the 27 other EU leaders, Mr Juncker said: "I don't like Brexit because I would like to be in the same boat as the British.

"The day will come when the British will re-enter the boat, I hope.

"But Brexit is not the end of the European Union, nor the end of all our developments, nor the end of our continental ambitions."

Theresa May was not present for the talks in Brussels, with the leaders of the other 27 nations considering the future of the bloc after Brexit.

With the Prime Minister expected to trigger Article 50 within days, the meeting was the last chance for the leaders to gather together before the formal Brexit process begins.

German Chancellor Angela Merkel confirmed that if Mrs May invokes Article 50 next week, there would be an extraordinary meeting of the EU 27 on April 6.

If Article 50 is triggered later this month, the meeting would be pushed back to later in April.

"We are well-prepared and we shall wait with interest, but which day in March it will be is not of such prime importance," she said.

Brexit had been a "wake-up call" to the other EU members to prepare for the future, she said at a press conference.

European Council president Donald Tusk said the EU would be ready to respond within two days of Mrs May triggering Brexit, amid speculation that she could invoke Article 50 on Tuesday if the Brexit Bill clears Parliament on Monday, when MPs are expected to overturn Lords amendments.

Further time has been allocated later in the week for a possible session of "parliamentary ping-pong" if peers insist on reinstating their amendments, but if they do not Mrs May could have the legal authority to fire the Brexit starting gun on Tuesday, when she makes a Commons statement on the Brussels summit.

Mr Tusk said: "We are well-prepared for the whole procedure and I have no doubt that we will be ready in 48 hours.

"I think it's a proper time to react and all of us, I mean member states and the European Council as a political body, we trust in our chief negotiators Jean-Claude and Michel Barnier."

Mr Juncker and Mr Tusk also called for unity among the 27 ahead of talks .

They both stressed the need for political ties to be maintained in the face of Brexit, despite recent calls from Germany, France and Italy for an EU which allows different countries to advance at their own pace.

Mr Juncker, a federalist, called on Europeans to "show their colours", before appearing to warn against a so-called multi-speed Europe : "I'm absolutely convinced that those who sit back on their laurels go a lot less far than those that run."

The comments appeared to put him at odds with Mrs Merkel, who described a multi-speed Europe as a "lived reality".

"We talked at some length about the so-called Europe of different speeds," she said.

"Some people voiced concerns that this might mean there are different classes of Europeans - first and second-class citizens, as it were.

"I said that such a difference in European unity is, in a way, laid down in the treaties. It is a lived reality now."

Mr Tusk, who was elected to serve a second term on Thursday, said: "It is clear from the debate that the unity of the 27 will be our most precious asset."

He said "the idea of a multi-speed Europe will be one of the discussions" ahead of a declaration on the future of the EU to coincide with the 60th anniversary of the Treaty of Rome, which laid the foundations for the union, later this month.

Acknowledging the differences within the bloc, Mr Tusk said: "Some expect systemic changes that would loosen intra-EU ties and strengthen the role of nations in relation to the community.

"Others, quite the opposite, are looking for new, deeper dimensions of integration, even if they would apply only to some member states.

"Such a possibility is indeed foreseen in the treaties currently in force.

"However, considering the interests of the community of 27 countries in the context of the upcoming Brexit negotiations, as well as the long-term strategic interests of the EU, I will be urging everyone to strive towards maintaining political unity among the 27."

Irish Taoiseach Enda Kenny said it would be a "political challenge" to ensure that the open border between Northern Ireland and the Republic is maintained after Brexit, even if the UK quits the European customs union.

Speaking to reporters as he left the summit, Mr Kenny said: "What we know is that I have agreed, the British Government has agreed and the British Prime Minister has agreed that we are not going back to a hard border.

"That is a political challenge. It is not beyond the ingenuity and the imaginative and creative capacity of both Irish and British officials to deal with that problem."

Asked whether he believed Britain would have to pay an exit bill of as much as 60 billion euros (£52 billion), Mr Kenny said: "I've never mentioned a figure in that regard. What is involved here is a principle and a methodology and out of that will come a figure.

"That will have to be discussed and negotiated, and that's what will happen."