British Prime Minister Theresa May says she will formally begin the Brexit process by the end of March 2017, in an interview with the BBC.

Speaking on the Andrew Marr Show, she did not give an exact date beyond saying "by the end of March" for triggering Article 50 in the Lisbon treaty of the EU needed by a member state to exit the bloc.

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 interview with the Sunday Times, May said that she is to put a new law in next spring before the British Parliament to repeal the act passed in 1972 that took the country into the EU.

The so-called "Great Repeal Act", to be announced in the forthcoming Queen's Speech at the official opening of Parliament, will end the supremacy of all European laws in the British courts.

Members in both the House of Commons and the House of Lords, who are highly split over the Brexit issues, will be forced to vote on the bill.

May told the Sunday Times that the measure would make Britain a "sovereign and independent country" again.

May announced the measures as thousands of Conservatives gathered in Birmingham on the first day of the party's annual conference.

Brexit is to dominate the first day of conference business, with a planned major speech by May Sunday afternoon.

Conservative MPs and peers are divided on whether there should be a hard Brexit or a so-called soft Brexit that could mean Britain having to follow some of Brussels' regulations as part of a new trade deal.

Brussels has insisted that if Britain wants to arrange a single-market deal with the EU, borders must be kept open for all European nationals.

However the issue of open immigration and uncontrolled access to Britain was one of the key drivers for the June 23 referendum decision when Britain voted by 52 percent to 48 percent to quit the EU.