This was done by a process known as 'royal assent'
Queen Elizabeth II has signed the Brexit bill in Britain, paving the way for Prime Minister Theresa May to trigger Article 50.
Officially known as the European Union (Notification of Withdrawal) Bill, it passed the UK parliamentary process following votes in the Houses of Commons and Lords on Monday.
Now that it has become law, Mrs May is free to decide when to invoke the two-year negotiation period for Britain to leave the European Union.
Downing Street has suggested this is not likely to happen until the last week of March.
Queen Elizabeth approved the bill by a process known as 'royal assent'.
This is when a monarch formally agrees to make a bill into an Act of Parliament (law).
When royal assent has been given, an announcement is made in both houses by its respective speakers.
The legislation within the bill can come into effect immediately, after a set period or after a commencement order by a government minister.
But according to British government rules, if there is no commencement order the act will come into force from "midnight at the start of the day of the royal assent."
It has not been refused since 1707, when Queen Anne refused it for a bill for settling the militia in Scotland.
The bill's passing into law follows a difficult day for the British government, in which ministers were accused of "driving towards a cliff-edge with a blindfold on".
Brexit Secretary David Davis was forced to admit his department has not made an assessment of the economic implications of failure to secure an agreement with the rest of the EU.
His admission was despite Mrs May repeatedly saying she thinks no deal is better than a bad deal.
Additional reporting: IRN