House of Commons Speaker John Bercow announced that the European Union (Notification of Withdrawal) Bill was signed by the Queen after it was cleared by both chambers of the Parliament earlier this week, the Guardian reported.
"I have to notify the house in accordance with the Royal Assent Act 1967 that her Majesty has signified her royal assent to the following acts... European Union (Notification of Withdrawal) Act 2017," Bercow told lawmakers.
The bill gives May the legal authority to trigger Article 50 of the Lisbon Treaty, the laid down procedure within the EU for a member state wishing to leave.
Once Article 50 is triggered, it kickstarts a two-year process for negotiating a new relationship between both sides.
The Prime Minister is now free to trigger the exit clause, but is not expected to do until as late as March 29, the newspaper reported.
A Downing Street spokesman said: "The legislation has been passed un-amended, we are pleased that's the case.
The legislation has got Royal Assent and we look forward to triggering negotiations by the end of March."
Brexit Secretary David Davis said: "The Queen has today given royal assent to the Article 50 bill, giving the government the formal power to it and deliver on the will of the people."
Indian-origin lawyer in Britain Sarosh Zaiwalla said: "The biggest headache for the Prime Minister now is the announcement by Scottish First Minister Nicola Sturgeon calling for a second referendum on the independence... If Scotland becomes independent, the UK will break up, and England will become little England. This has to be kept in mind."