The Queen of England, Queen Elizabeth II has granted royal assent to the European Union (Notification of Withdrawal) Bill, giving Theresa May the legal power to choose when to start formal Brexit talks.
The Bill was passed by parliament on Monday.
It will then be open to the prime minister to notify Brussels that the UK is leaving the EU, starting the two-year countdown to Brexit.
Mrs May promised on Tuesday to come to the Commons later this month to announce when she had triggered Article 50, the legal mechanism for withdrawal.
She revealed in October that it would be done by the end of March.