Theresa May has written to the European Union to request a further delay to Brexit until 30 June.
The UK is currently due to leave the EU on 12 April and, as yet, no withdrawal deal has been approved by MPs.
The prime minister has proposed that if UK MPs approve a deal in time, the UK should be able to leave before European Parliamentary elections on 23 May.
But she said the UK would prepare to field candidates in those elections in case no agreement is reached.
It is up to the EU whether to grant an extension to the Article 50 process, under which the UK leaves the EU, after MPs repeatedly rejected the withdrawal agreement reached between the UK and the bloc.
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BBC Europe editor Katya Adler has been told by a senior EU source that European Council President Donald Tusk will propose a 12-month “flexible” extension to Brexit, with the option of cutting it short, if the UK Parliament ratifies a deal.
But any proposal would have to be agreed unanimously by EU leaders when they meet on 10 April .
The prime minister wrote to Mr Tusk to request the extension ahead of that meeting.
She had already requested an extension to the end of June at the last summit, which took place shortly before 29 March – the date the UK was originally meant to have left the EU.
But she was offered a short delay to 12 April – the date by which the UK must say whether it intends to take part in the European Parliamentary elections – or until 22 May if UK MPs had approved the withdrawal deal negotiated with the EU. They voted it down for a third time last week.