Theresa May has written to European Council president Donald Tusk requesting an extension to Article 50 until June 30.
The Prime Minister said she will seek to ratify her Withdrawal Agreement before the European Parliament elections on May 23, but will make "responsible preparations" to take part if that does not prove possible.
Britain is due to leave the European Union at the end of next week, but Mrs May is now seeking to delay Brexit for a second time after her deal was rejected for a third time last week.
It comes amid reports that EU officials are considering offering the prime minister a 12-month flexible extension - or "flextension" - to the Article 50 timetable.
Mr Tusk is preparing to put the option to EU leaders at a crunch summit next Wednesday in a bid to prevent the UK crashing out of the bloc on April 12, according to the BBC.
The first page of Theresa May's letter to Donald Tusk.
In her letter, Mrs May wrote: "I am writing therefore to inform the European Council that the United Kingdom is seeking a further extension to the period provided under Article 50(3) of the Treaty on European Union, including as applied by Article 106a of the Euratom Treaty.
"The United Kingdom proposes that this period should end on 30 June 2019. If the parties are able to ratify before this date, the Government proposes that the period should be terminated early.
"The Government will want to agree a timetable for ratification that allows the United Kingdom to withdraw from the European Union before 23 May 2019 and therefore cancel the European Parliament elections, but will continue to make responsible preparations to hold the elections should this not prove possible."
Mrs May said if ongoing talks with Labour do not lead to a "single unified approach soon" then the Government would instead look to establish a "consensus" on options on a future relationship that could be put to the Commons.
She wrote: "The Government stands ready to abide by the decision of the House, if the Opposition will commit to doing the same."