Tories cheer at news Queen has given the go-ahead for Brexit
The Queen has signed the Article 50 Bill into law, giving Theresa May the power to start Brexit negotiations.
Tory MPs cheered as Commons Speaker John Bercow announced that the European Union (Notification of Withdrawal) had been given royal assent.
Brexit Secretary David Davis said Mrs May would formally invoke Article 50 by the end of March, allowing the UK to start talks on creating a "positive new partnership" with the EU.
The Article 50 process, the mechanism for withdrawal from the EU, involves a two-year timetable for the UK to leave the bloc.
Mr Davis said: "By the end of the month we will invoke Article 50, allowing us to start our negotiations to build a positive new partnership with our friends and neighbours in the European Union, as well as taking a step out into the world as a truly global Britain."
Britain's future trading relationship with the bloc and any exit bill which it may have to pay are set to be highly contentious issues in the forthcoming negotiations.
The Government managed to get the two-clause Bill through Parliament unamended despite opposition from the Lords, after a threatened Tory rebellion in the Commons fizzled out.
Labour has vowed to continue the battle to guarantee the rights of EU nationals living in the UK and give Parliament a meaningful vote on the deal, despite failing to get these pledges attached to the Bill and on to the statute book.
The party has tabled two new motions to be debated in the Lords at the end of this month.
Meanwhile, former Irish president Mary McAleese has said she is worried that if the Irish border hardens, hearts will harden too.
Dr McAleese expressed concern that Brexit may destroy the goodwill built up during the peace process.
In an interview with the Catholic weekly The Tablet, she added that Ireland has a "vested interest in ensuring that the relationship Britain maintains with the European Union is as strong as it can humanly be."
She stressed that membership of the EU played a huge part in improving British-Irish relations and that it was crucial at the time of the Good Friday Agreement in 1998. Without it, she said, there might have been a different outcome to the vote in support of the peace process.
Referring to that vote, Dr McAleese said: "I'm not sure if we had known at the time that Britain was going to pull out of the European Union and with it bring Northern Ireland out... that people south of the border would have been as quick to sign off on changing the Irish constitution, because it was easier to do that [as] we were all members of the European Union."
She added: "It was an easier sell and nobody thought in terms of withdrawing from the EU, so now we are in a very, very different situation."