European Council president Donald Tusk said he still believed that Brexit could be reversed after the UK's departure from the EU was delayed until October 31.
Mr Tusk told MEPs that the UK would continue to be represented in the European Parliament for "several months - maybe longer".
The prospect of the UK taking part in the May 23 elections, almost three years after voting to leave the European Union, has fuelled anger among Brexiteers.
Theresa May secured the extension to the Brexit process at a summit last week in order to prevent a no-deal split from the EU, but still hopes the UK can leave earlier than the Halloween deadline if she can get a withdrawal agreement through the Westminster Parliament.
Speaking in Strasbourg, Mr Tusk said he disagreed with one of the EU leaders who had said the EU should abandon hope of the UK reversing its decision to leave.
"During the European Council one of the leaders warned us not to be dreamers, and that we shouldn't think that Brexit could be reversed," he said.
"I would like to say: at this rather difficult moment in our history, we need dreamers and dreams.
"We cannot give in to fatalism. At least I will not stop dreaming about a better and united Europe."
European Parliament chief Brexit co-ordinator Guy Verhofstadt expressed his fear that Brexit will "poison" the European elections.
Referring to the delay to Brexit, he said: "I fear that it will continue the uncertainty. I fear that it will prolong the indecision. And I fear most of all that it will import the Brexit mess into the European Union.
"And moreover, that it will poison the upcoming European election."
Brexit Party leader Nigel Farage claimed his party would "sweep the board" at the elections on May 23 - and would win a general election if Mrs May and Mr Corbyn agree to the "betrayal" of a permanent customs union.
He told the European Parliament: "The Brexit Party will sweep the board in these elections and there is only one way it can be stopped and that is if the governing party of Mrs May and the opposition of Mr Corbyn come together and agree to a permanent customs union, and indeed effectively membership of the single market.
"If that happens, the Brexit Party won't win the European elections but it will win the general election because the betrayal will be so complete and utter, so I don't believe it's going to happen."