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A bill authorizing Britain to start its exit from the European Union has received royal assent and is now law.

House of Commons Speaker John Bercow announced Thursday that the European Union (Notification of Withdrawal) Bill has received the assent of Queen Elizabeth II.

The bill was passed by Parliament on Monday.

Royal assent is a formality — no monarch has refused to sign a bill for more than 300 years

It means that Prime Minister Theresa 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.