The UK will begin the formal Brexit negotiation process by the end of March next year, British Prime Minister Theresa May has said.

Photo: AFP

The timing on triggering Article 50 of the Lisbon Treaty means the UK looks set to leave the EU by summer 2019.

Britain's shock vote to leave the European Union in June propelled Mrs May to power and the prime minister has been under pressure to offer more details on her plan for departure, beyond an often-repeated catchphrase that "Brexit means Brexit".

Mrs May told the Tory Party conference - her first as prime minister - the government would strike a deal with the EU as an "independent, sovereign" UK.

Voters had given their verdict "with emphatic clarity", she said, and ministers had to "get on with the job".

In a move to ease fears among Conservatives that she may delay the exit, Mrs May told the party's conference in Birmingham she was determined to move on with the process and win the "right deal".

She said the UK was going to be a fully independent, sovereign country which was no longer part of a political union with supranational institutions that could override national parliaments and courts.

"And that means we are going, once more, to have the freedom to make our own decisions on a whole host of different matters, from how we label our food to the way in which we choose to control immigration."

She said a "truly global Britain is possible, and it is in sight", adding: "We don't need - as I sometimes hear people say - to 'punch above our weight' because our weight is substantial enough already."

Mrs May, who had previously only said she would not trigger Article 50 this year, ended speculation about the government's timetable, telling the BBC earlier that it would be done by "the first quarter of 2017", marking the start of a two-year exit process.

Mrs May said the process of leaving the EU would be "quite complex" but 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".

Scottish First Minister Nicola Sturgeon said it was "depressing" that government decisions were "being driven by ideology of the hard Brexiteers, rather than interests of country".

Using Article 50 of the EU's Lisbon Treaty will give Britain a two-year period to clinch one of the most complex deals in Europe since WWII.

European Council President Donald Tusk said the announcement brought "welcome clarity", but behind the scenes, there was frustration at the lack of detail.

"It is beyond comprehension that the politicians who campaigned for Brexit for months have no idea what they want, they have no plan at all," a senior German official said.

- BBC / Reuters