Theresa May has said she “sincerely hopes” the UK will leave the EU with a deal and she is still “working on” ensuring Parliament’s agreement.
Arriving in Brussels, she said that she had “personal regret” over her request to delay Brexit, but said it will allow time for MPs to make a “final choice”.
At the EU summit she will try to persuade the other 27 countries to delay the UK’s exit beyond 29 March.
Meanwhile, Jeremy Corbyn said his talks in Brussels were “very constructive”.
On Wednesday, Mrs May made a speech blaming the delay on MPs and telling the nation she was “on their side”.
The BBC’s Europe correspondent Kevin Connolly said the EU side “appears to hold all the aces” at the summit, with some countries including France “sceptical of the value of making an offer” of an extension.
