Theresa May is briefing the House of Commons on her agreement with EU leaders to delay Brexit by more than six months.
The new deadline of 31 October means the UK is likely to have to hold European Parliament elections in May.
The prime minister promised to pursue an “orderly” Brexit, saying it was her “priority” to deliver it.
Earlier, Labour welcomed the delay, promising to work with the government to “break the impasse” in Parliament.
Brexit was originally set to happen on 29 March. But after MPs repeatedly rejected Mrs May’s withdrawal agreement with the EU, the deadline was put back to 12 April.
The new 31 October deadline – agreed after talks in Brussels went on late into the night – averts the prospect of the UK having to leave the EU without a deal this Friday.
Under EU rules. the latest delay means the UK will have to hold European Parliament elections in May, or face leaving on 1 June without a deal.
Mrs May said she “profoundly” regretted her deal not being agreed to by MPs.
She said: “The whole country is intensely frustrated that this process of leaving the European Union has not been completed.”
On the latest delay, she said: “The choices we face are stark and the timetable severe.”
Talks with Labour
Before the Brussels summit, Mrs May had told leaders she wanted to move the UK’s exit date from this Friday to 30 June, with the option of leaving earlier if Parliament ratified her agreement.
The government is continuing to hold talks with Labour aimed at reaching a consensus on how to break the deadlock in Parliament.
For Labour, shadow Brexit secretary Sir Keir Starmer called the delay to 31 October “a good thing”, saying businesses would be “relieved”.
He added: “Negotiations are in good faith. We all feel a deep sense of duty to break the impasse.
“But there’s also this question of how on Earth do we ensure that anything this prime minister promises is actually delivered in the future because of course she’s already said she’s going to step down, probably within months.”
What was agreed?
- A Brexit extension “only as long as necessary” and “no longer than 31 October” to allow for the ratification of the withdrawal agreement
- The UK “must hold the elections to the European Parliament” and if it fails to do this, the UK will leave on 1 June
- The European Council reiterates there can be no reopening of the withdrawal agreement negotiations
- Read the EU’s conclusions here.
- What is Theresa May’s withdrawal agreement?
European Council President Donald Tusk said future developments were “entirely in the UK’s hands”, adding: “They can still ratify the withdrawal agreement, in which case the extension can be terminated.”
Mr Tusk said the UK could also rethink its strategy or choose to “cancel Brexit altogether”, but urged: “Please do not waste this time.”
The EU had been split over the length of delay to offer the UK, and by law its other 27 member states had to reach a unanimous decision.
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