The Prime Minister revealed Article 50 of the Lisbon Treaty will be triggered before March 2017.
This will give Britain two years to negotiate the terms of Brexit.
Mrs May spoke on the BBC’s Andrew Marr Show on Sunday morning ahead of the Conservative Party conference in Birmingham.
Britain officially voted to leave the EU in the referendum held in June.
The PM said: “I’ve been saying we wouldn’t trigger it before the end of this year so we can get some preparation in place.
“But yes we will trigger (Article 50) before the end of March next year.”
The two-year process can be extended beyond two years if Britain and all other EU nations unanimously agree.
Mrs May also spoke of her plans for a “Great Repeal Bill”, which will set all existing EU law into UK law ahead of the day the country leaves the union in the summer of 2019.
This will be the first time in history Article 50 has been triggered.
Mrs May added: “The remaining members of the EU have to decide what the process of negotiation is.
“I hope, and I will be saying to them, now they know what our timing is going to be we’ll be able to have some preparatory work so that once the trigger comes we have a smoother process of negotiation.
“It’s not just important for the UK but important for Europe as a whole that we’re able to do this in the best possible way so we have the least disruption for businesses.
“Then also when we leave the EU we have a smooth transition from the EU.”
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