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 formal mechanism for withdrawal from the EU, involves a two-year timetable for the UK to leave the bloc.
Mr Davis said: "The Queen has today given royal assent to the Article 50 Bill, giving the Government the formal power to trigger Article 50 and deliver on the will of the British people.
"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 both 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 House of 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.
The first, tabled by Baroness Hayter of Kentish Town, would require a minister to report back to peers at an early stage on the progress being made in guaranteeing the rights of EU nationals.
The second, tabled by Labour Lords leader Baroness of Smith of Basildon, demands the establishment of a joint committee of the Commons and the Lords "to consider and report on the terms and options" for a vote in Parliament on the Brexit deal.
Announcing news of the royal assent to MPs on the Commons, Mr Bercow said simply: "I have to notify the House in accordance with the Royal Assent Act 1967 that Her Majesty has signified her royal assent to the following acts: Supply and Appropriation (Anticipation and Adjustments) Act 2017, European Union (Notification of Withdrawal) Act 2017."
Labour MP Gisela Stuart, who backed Brexit and chaired Vote Leave, said: "This is a simple yet significant step in implementing the referendum result and respecting the decision of the British people to leave the EU.
"Brexit will allow us to begin the process of national renewal, enabling us to build a robust economy, more cohesive communities and to make politicians more accountable to the public.
"In the weeks and months ahead, I would urge politicians from all parties to work together to help ensure we make the most of the opportunities that leaving the EU will provide."