Talks over Britain's departure from the EU should be as short as possible to avoid prolonged uncertainty, prime ministerial candidate Andrea Leadsom said on Monday as she launched her leadership bid.

Setting out her stall as the "Leave" alternative to frontrunner Theresa May, Leadsom said it was in the interests of EU countries to do a tariff-free trade deal with Britain, without the UK having to allow continued free movement of people.

"Democratically elected European parliaments will take pragmatic decisions about what's in their interest and they will choose to give tariff-free access to the UK," she added.

"That will not be, I don't think, a bargaining chip around free movement."

Leadsom, 53, second-favourite in the race to succeed David Cameron, was a leading figure in the campaign to leave the EU. Her position differs from May, who backed the "Remain" campaign and believes Article 50, which starts the formal process of EU withdrawal, might not be invoked until next year.

Leadsom has said she will trigger Article 50 as quickly as possible if she wins.

"I intend to keep the negotiations as short as possible," she told reporters on Monday. "Neither we, nor our European friends, need prolonged uncertainty and not everything needs to be negotiated before Article 50 is triggered, and the exit process is concluded."

She said she would guarantee the rights of EU nationals living in Britain, something that May has not done.

Leadsom, a former energy minister and Economic Secretary to the Treasury, joined other candidates in ruling out a new general election following the leadership contest.

She also said she would keep the United Kingdom together despite calls in pro-EU Scotland for a second independence referendum after last month's EU vote.

Conservative lawmakers will gradually whittle down the five contenders to just two, starting on Tuesday, before grassroots party members vote in early September to decide the winner.