Donald Tusk has told MEPs a long extension to the Brexit process allows for “all options” to remain on the table, including the possibility of a second referendum.
In reference to claims from Brexiteers such as Jacob Rees-Mogg in recent weeks, Mr Tusk added: “I know that some have expressed fear that the UK might want to disrupt the EU‘s functioning during this time.
“But the EU did not give in to such scaremongering... in fact, since the very beginning of the Brexit process the UK has been a constructive and responsible EU member state. So we have no reason to believe that this should change.”
The European council president also warned those opposed to Brexit not to "give in to fatalism" by accepting the UK's departure from the bloc, as he said both Europe and Britain needed "dreams and dreamers".
“During the European Council one of the leaders warned us not to be dreamers and that we shouldn’t think that Brexit can be reversed,” Mr Tusk told MEPs in Strasbourg.
His remarks came as Jean-Claude Juncker, the European Commission president, said the EU had "nothing to gain" from the disruption a no-deal Brexit would bring to the UK.
He said he had adopted the "necessary contingency measures", but said only those who seek to undermine the global legal order would benefit from such an exit. "We have adopted the necessary contingency measures and we are ready for a no-deal Brexit," he told MEPs.
"But our union has nothing to gain from great disruption in the United Kingdom. The only ones who would benefit are those who resent multilateralism and seek to undermine the global legal order."
Mr Juncker made the comments as he addressed the European Parliament in Strasbourg on last week's European Council summit at which Theresa May was offered a six-month Brexit delay.
Elsewhere, it also emerged that more than 3,000 people had applied to stand as candidates for Change UK during the European Parliament elections on 23 May, as it was officially registered as a political party by the Electoral Commission.
This liveblog has now closed, but you can see Tuesday's updates below
"Second, it allows the EU to focus on other priorities that are at least as important, such as trade with the US or the new EU leadership.
"I know that some have expressed fear that the UK might want to disrupt the EU's functioning during this time. But the EU did not give in to such scaremongering... in fact, since the very beginning of the Brexit process the UK has been a constructive and responsible EU member state. So we have no reason to believe that this should change.
"Third, the flexible extension delays the possibly of a no-deal Brexit by six months. Thanks to this millions of people and businesses have gained at least some certainty in this unstable time."

Theresa May ‘must appoint deputy’ to oversee next Brexit stage to avoid blunders of first phase
Theresa May must appoint a powerful “deputy” with agreed aims for the next stage of the Brexit negotiations to avoid the blunders of the first phase, a major study says.
The government is in danger of “stumbling” into future trade talks with the EU “without a plan”, it warns – even if the divorce deal is eventually approved by the Commons.
The warning comes as compromise talks between the government and Labour stalled, casting fresh doubt on the prime minister’s hopes of passing her deal and avoiding a long Brexit delay.
"We have adopted the necessary contingency measures and we are ready for a no-deal Brexit," he said at the European Parliament in Strasbourg.
"But our union has nothing to gain from great disruption in the United Kingdom. The only ones who would benefit are those who resent multilateralism and seek to undermine the global legal order."
"We should approach this seriously as UK members of the European Parliament will be there for several months - maybe longer.
"They will be full members of the Parliament with all the rights and obligations. I am speaking about this today because I have strongly opposed the idea that during this further extension the UK should be treated as a second category member state. No, it cannot.
"Therefore I also ask you to reject similar ideas if they were to be voiced in this House."
He told the European Parliament: "The Brexit Party will sweep the board in these

EU president says Brexit can be stopped and that 'we cannot give into fatalism'
The president of the European Council has warned opponents of Brexit not to “give in to fatalism” and accept Britain’s departure from the European Union.
Speaking in the European Parliament on Tuesday Donald Tusk said Europe and Britain needed “dreams and dreamers” to keep the idea of a united Europe alive and Britain in the EU.
“During the European Council one of the leaders warned us not to be dreamers and that we shouldn’t think that Brexit can be reversed,” Mr Tusk told MEPs in Strasbourg.

UK unemployment stays at lowest level since 1975
UK unemployment kept at lowest level since 1975, official figures have shown.
The jobless rate remained at 3.9 per cent in the three months to the end of February, the Office for National Statistics said on Tuesday.
An estimated 1.34 million people were unemployed between December 2018 and February 2019, a fall of 76,000 from a year earlier.
He added: "The government doesn't appear to be shifting the red lines because they've got a big pressure in the Tory party that actually wants to turn this country into a deregulated low-tax society which will do a deal with Trump.
"I don't want to do that."

Blair and Ahern call for second Brexit referendum to protect Good Friday Agreement
Former prime minister Tony Blair and former Irish taoiseach Bertie Ahern have called for a second Brexit referendum, in a joint editorial emphasising the importance of the Good Friday Agreement.
Writing in The Irish Times, the co-signatories of the 1998 accord which brought peace and power-sharing to Northern Ireland, warn that Brexit is “the most serious threat to the Belfast Agreement since it was created and to the union in our lifetime".
Mr Blair and Mr Ahern stress that alongside the principle of consent on Northern Ireland’s constitutional status, the agreement acknowledged a “nationalist aspiration to have an open border between North and South”, which Brexit now threatens.
"You can see the majesty and beauty of that building and to see it destroyed is devastating, I think for everybody in Paris and indeed around the world, because you see beautiful buildings like that and think of the beautiful buildings we've got in this country. If any of those were destroyed in fire how would we feel about it?
"Well done on the firefighters on at least managing to contain it in the end, but sadly the spires have gone and I note that President Macron wants to rebuild the whole cathedral. I hope that happens."
He said: "The state of the building is very poor in Westminster and a fire risk is obviously huge with a building that has so much wood within it."

Has Nigel Farage killed Ukip with the Brexit party?
Brexit Explained: With the launch of a new Eurosceptic party, Ukip’s former leader may have dealt his old party a fatal blow, Europe Correspondent Jon Stone writes.
Opinion: Victory awaits Corbyn if he can seize the day for Remain
Brexit and the long recession that will follow is surely an obstacle to a socialist society. In the European election manifesto, Corbyn will finally have to say whether Labour is a Remain party, writes Sean O'Grady“Change UK – The Independent Group have a clear message in the European elections: we demand a People’s Vote and, if it is held, all our MPs will campaign to remain in and reform the European Union."

Ukip MEPs quit to join Nigel Farage’s Brexit Party
Eurosceptic party’s attitude to women was ‘disgusting’, Jane Collins says
Nancy Pelosi: No UK-US trade deal if Good Friday Agreement is undermined
Speaker of the US House of Representatives says peace treaty could not be 'bargained away in another agreement'