A bill authorizing Britain to start its exit from the European Union received royal assent and became law Thursday, empowering Prime Minister Theresa May to begin divorce proceedings from the bloc. House of Commons Speaker John Bercow announced that the European Union (Notification of Withdrawal) Act has received the assent of Queen Elizabeth II. The bill was passed by Parliament on Monday but needed royal assent to become a law. Such assent is a formality — no monarch has refused to sign a bill for more than 300 years. May is now free to invoke Article 50 of the EU’s key treaty, triggering two years of exit negotiations. She says she will do that by March 31, calling it a “decisive moment” for Britain. The prime minister’s office has indicated notification will come in the last week of the month. Under the terms of the EU treaty, Britain will no longer be a member of the bloc two years after May issues the notification. The British government hopes to seal a new free-trade deal between the U.K. and the bloc in that time. But EU negotiators warn it could take two years just to settle the divorce terms; agreeing on a new relationship for the U.K. and the EU could take years longer.