Theresa May will make a direct plea to EU leaders later asking to postpone Brexit for three months, hours after telling the British public a delay was “a matter of great personal regret”.
At an EU summit in Brussels, she will try to persuade the other 27 countries to delay the UK’s exit beyond 29 March.
On Wednesday, the PM made a speech blaming the delay on MPs and telling the nation she was “on their side”.
Meanwhile, Jeremy Corbyn is also due in Brussels for separate Brexit talks.
EU leaders are expected to arrive at the two-day summit from 13:00 GMT.
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.
How the day will unfold (timings are approximate):
13:00 GMT – Theresa May arrives in Brussels
14:30 GMT – Mrs May will make a short speech to the 27 EU leaders, making the case for a delay to Brexit
The PM will then leave the room while the EU leaders decide whether to grant the UK’s request and discuss other Brexit options
18:00 GMT – Press conference by European Council President Donald Tusk and European Commission President Jean-Claude Juncker. Mrs May is also expected to make a statement
18:30 GMT – Working dinner
Speaking in the German Parliament, Angela Merkel said the EU could meet Mrs May’s request to delay Brexit if in the next week there was a “positive vote” on the withdrawal agreement in the UK Parliament.
The German Chancellor said European elections at the end of May would have to be considered during discussions on the suggested extension deadline of 30 June, adding: “But of course we can certainly talk about a short term extension.”
The UK is set to leave the EU next Friday unless the law is changed. The current default position for leaving is without a withdrawal agreement.
Mrs May agreed a deal with the EU, but MPs have rejected it twice.
She has asked the EU for a short extension of the two-year Brexit process until 30 June, but any extension needs to be agreed to by all EU members.
European Council President Donald Tusk said he believed the EU would agree to a short extension, but this would only be if Mrs May’s deal is signed off by MPs next week. Another EU summit next week could be called in an emergency if needed, he said.
Mr Tusk said the “question remains open” as to how long a delay the other EU leaders would support.