Prime Minister Theresa May said on Sunday that Britain would start the formal process for leaving the European Union by the end of March 2017.
Before now May has only said that Britain would not trigger Article 50 of the EU's Lisbon Treaty -- which sets a maximum two-year clock ticking until a country's departure from the 28-member bloc -- before the end of this year.
May said she would be giving further details during her speech on Sunday to her governing centre-right Conservative Party's on the opening day of its annual conference in Birmingham, central England.
"I've been saying that we wouldn't trigger before the end of this year so that we get the preparation in place," she told BBC television.
"We will trigger before the end of March next year."
Earlier she announced a "Great Repeal Bill" to end the authority of EU law once Britain leaves the union.
The legislation will overturn laws that make EU regulations supreme, enshrine all EU rules in domestic law and confirm the British parliament can amend them as it wants.
"This marks the first stage in the UK becoming a sovereign and independent country once again," May told The Sunday Times newspaper.
"It will return power and authority to the elected institutions of our country. It means that the authority of EU law in Britain will end."