Prime Minister Theresa May on Tuesday said Britain will leave the EU's single market in order to restrict immigration in a clean break from the bloc, but lawmakers can vote on the final deal.

"Brexit must mean control of the number of people coming from Europe, and that is what we will deliver. What I am proposing cannot mean membership of the single market," May said during a highly-anticipated speech at London's Lancaster House.

She added that Britain would seek a trade deal giving "the greatest possible access" to the market on its departure.The prime minister also announced that any divorce deal with the remaining EU members must be approved by votes in both chambers of Britain's parliament.

Britain has two years to negotiate a break-up deal once May triggers Article 50 of the Lisbon Treaty, officially declaring the country's intention to quit, or face leaving with no agreement.  

May has promised to trigger Article 50 by the end of March, and said that she believed a final settlement and trade deal could be simultaneously negotiated within the timeframe.

Foreign partners doubt such a timetable, with Austrian Foreign Minister Hans Joerg Schelling saying Brexit would take five years.

"It is not clear if it will be possible to negotiate the UK's exit from the EU and the terms of a future cooperation at the same time," he said.

Seeking to calm fears of a sudden jolt to the economy on abruptly leaving the EU, May said she would seek a "phased process of implementation".

Her direction will be cheered by those who want to leave the EU, but dismay those who fear the impact on Britain's economy.

EU countries accounted for 44 percent of Britain's total exports in goods and services in 2015, with the country recording a ?68.6 billion ($82.7 billion, 77.9 billion euros) trade deficit with the bloc. (AFP)