The Queen signed the Bill into law after it was approved in the House of Commons and the House of Lords on Monday.
The Royal Assent was presented to both House by their speakers and is now an Act of Parliament.
The PM can now decide for herself when to trigger Article 50 of the Lisbon Treaty, initiating the two-year process of extracting the UK from the bloc.
May has signaled the bill will be triggered at the end of March in order to avoid overlaps with next week’s informal meeting between EU countries.
The meeting will mark the 60th anniversary of the Treaties of Rome, which saw the foundation of the European Economic Area, which in turn led to the European Union.
May, however, said she will not be attending the meeting on March 25.
The Royal Assent comes a day after Brexit Secretary David Davis admitted there has been no assessment of how the UK may suffer if it fails to reach a deal with the EU.
May has previously said “no deal is better than a bad deal,” but the Treasury has warned that crashing the UK out of the EU without a bargain could lead to economic collapse.
Davis, however, shrugged off such concerns, telling the House of Commons that “it’s not as frightening as some people think, but it’s not as simple as some people think.”