The government will put the official Brexit date and time into law in an amendment to the EU Withdrawal Bill, in what it said was a bid to end "confusion or concern" over the exact schedule.
The Department for Exiting the EU this evening announced "exit day" will be 11pm GMT on 29 March 2019.
This means Brexit will be an hour earlier than previously thought. A government spokesperson had previously said the UK will leave the EU "when Big Ben bongs midnight" on 29 March.
The new date will be included on the face of the withdrawal bill, and will remove the power of ministers to set the exit date themselves, meaning a new act of Parliament would be needed to extend the Article 50 period of negotiations, if the EU were to agree.
The announcement comes with the government in crisis mode, after the resignation of two ministers within a week, as well as the ongoing fall-out from the sex scandal gripping parties across Westminster.
The move will likely placate some Brexit-supporting backbench MPs who believe a delay in the leaving date could allow their political opponents to frustrate Brexit.
Over the last two days Prime Minister Theresa May came under pressure to replace former international development secretary Priti Patel, who resigned over unsanctioned meetings with Israeli politicians, with another Brexit supporter. May duly appointed Penny Mordaunt to the vacated post.
Brexit secretary David Davis said: "Our amendment makes it crystal clear that the UK is leaving the EU at 11pm on 29 March 2019. We’ve listened to members of the public and Parliament and have made this change to remove any confusion or concern about what ‘exit day’ means.
Davis added he will "work with" the opposition to amend the bill.
He said: "This important step demonstrates our pragmatic approach to this vital piece of legislation. Where MPs can improve the Bill, whatever their party, we will work with them.
“We look forward to further debate in the House of Commons when committee stage begins next week."