Theresa May will formally begin the Brexit process by the end of March 2017, she said.
The prime minister confirmed the deadline for triggering Article 50 of the Lisbon Treaty, which sets in place a two-year process of withdrawal.
She has also promised a “Great Repeal Bill” in the next Queen’s Speech, which will overturn the act that took the UK into the forerunner of the EU.
It will remove the European Communities Act 1972 from the statute book.
The government will also enshrine all existing EU law into British law.
The repeal of the 1972 Act will not take effect until the UK leaves the EU under Article 50.
Mrs May, who had previously only said she would not trigger Article 50 this year, will be making a speech on Brexit later to the Conservative conference, which is getting under way in Birmingham.
Speaking on the Andrew Marr Show, she did not give an exact date beyond saying “by the end of March” for triggering Article 50, but said it set the timetable “for the first quarter of 2017”.
Mrs May said she hoped there would now be “preparatory work” with the remaining EU members so that “once the trigger comes we will have a smoother process of negotiation”.
She added: “It is important for the UK and Europe as a whole to do this in the best possible way so there is a smooth transition (for the UK) away from the EU.”
In an earlier interview the prime minister said the repeal bill would mark “the first stage in the UK becoming a sovereign and independent country once again”.
“It will return power and authority to the elected institutions of our country,” she said.
“It means that the authority of EU law in Britain will end.”
Conservative Party chairman Patrick McLaughlin said Brexit negotiations would not be conducted in the public eye.
“You don’t say exactly what you are going to negotiate on, but once negotiations are concluded we will say what we’ve achieved and how we’ve achieved it,” he said.
Mrs May has also made it clear she does not want the party conference to be dominated by the issue of leaving the EU.
Tory MPs are divided between favouring a “hard Brexit” outside of the European single market to obtain complete control over immigration, or a “soft Brexit” where the UK remains in the free trade zone, but potentially has to comply with some EU rules.
Labour MP Phil Wilson, from the Open Britain campaign, said businesses wanted the prime minister to commit to single market membership.
“As car manufacturers have made clear, it is essential that the UK remains a member of the single market to protect investment and jobs.”
Elsewhere, Secretary of State for Exiting the European Union David Davis will tell the Conservative Party conference on Sunday that employment rights “will not be eroded” on exiting the EU.
He will dismiss any suggestion that the government intends to use Brexit to roll back workers’ rights and will say that UK law goes further than the minimum standards offered under EU law, such as for annual and parental leave.
He will also say: “The moment we leave, Britain must be back in control.
“To ensure continuity, we will take a simple approach. EU law will be transposed into domestic law, wherever practical, on exit day.
“It will be for elected politicians here to make the changes to reflect the outcome of our negotiation and our exit.”