Theresa May has confirmed Article 50 of the Lisbon Treaty, which triggers the formal withdrawal process for Brexit, will be invoked by the end of March 2017.
It will mark the start of the two-year process to enact Britain's withdrawal from the European Union.
The prime minister made the announcement after revealing plans for a "Great Repeal Bill" which will overturn legislation that took Britain into the EU over 40 years ago.
She told the BBC's Andrew Marr Show: "As you know, I have been saying that we wouldn't trigger it before the end of this year so that we get some preparation in place.
"But yes, I will be saying in my speech today (at the Tory conference) that we will trigger (Article 50) before the end of March next year."
- The 'Great Repeal Bill'
Mrs May said the introduction of a "Great Repeal Bill" was an "important step" because it makes it very clear to the British people who voted to leave the Government's intentions.
She added that it also provides a "greater degree of clarity" about timetables being followed.
"It's important for us to set this out now so that we have the timing, so that when we leave the European Union there is a smooth transition. I think that's important for us all, for particularly for the economy and for business."
What will it involve?
- The European Communities Act, which legislated for the UK's accession to the Common Market in 1972, will be removed from the Britain's statute book.
- All EU regulations will then be converted into domestic law, meaning Parliament can pick and choose which laws should be kept or revoked.
- The Bill is also designed to give certainty to businesses and protection for workers' rights that are part of EU law.
- It will also mean rulings by the European Court of Justice will stop applying to the UK once the legislation takes effect.
Reacting to Mrs May's announcement, former Tory minister Anna Soubry, who had campaigned for Britain to remain in the EU, said triggering Article 50 so soon "really concerns" her.
She also warned that the EU "holds all the cards" in the negotiation.
Labour shadow minister Jon Ashworth called for more clarity over the Government's Brexit strategy.
"Theresa May said she was providing 'clarity' but that's exactly what we aren't getting from the Tories," he said.
"She gave very little detail on her supposed big idea of a 'Great Repeal Act' other than it's an ambition; there was no answer on what would be in it, how it would work or, vitally, how she intends to deliver Brexit while protecting our workers and businesses."
Brexit Secretary David Davis is expected to tell the Conservative Party conference: "To those who are trying to frighten British workers, saying 'when we leave, employment rights will be eroded', I say firmly and unequivocally, 'no they won't'".
"As we prepare for those negotiations in Europe, we also need to prepare for the impact of Brexit on domestic law", Davis will say.
"It's very simple. At the moment we leave, Britain must be back in control. And that means EU law must cease to apply.
"To ensure continuity, we will take a simple approach. EU law will be transposed into domestic law, wherever practical, on exit day".