London: Theresa May, the frontrunner to replace British Prime Minister David Cameron, is "bloody difficult" without much experience in foreign affairs, a senior Conservative lawmaker was caught on camera saying on Tuesday. He also described Michael Gove, another leadership contender, as "wild" and the idea that Boris Johnson might have become prime minister as "ridiculous".

Ken Clarke, 76, a minister in the governments of Conservative leaders Cameron, John Major and Margaret Thatcher, criticised several of the candidates for his party's leadership in a conversation with another former minister broadcast by Sky News.

He was speaking as Conservative lawmakers began voting in the contest to replace Cameron, who said he would step down after Britons voted last month to leave or "Brexit" the European Union.

Ken Clarke in a candid conversation with Sir Malcolm Rifkind in the Sky News studio. Photo: Screenshot, Sky News

"Theresa is a bloody difficult woman but you and I worked for Margaret Thatcher... She won't be any more difficult than that," he said in the clip. It was not clear if he knew he was being filmed.

Mrs May has been interior minister for the last six years, the longest period of any politician for a century in a job which is often described as one of the cabinet's most challenging roles.

"I get on all right with her and she is good," said Mr Clarke, adding that he expected he would "wind up voting Theresa" after possibly lending his vote to work and pensions minister Stephen Crabb in the first round.

Britain's Home Secretary Theresa May is considered the most likely next UK prime minister. Photo: AP

"She's been at the Home Office (interior ministry) far too long, so I only know in detail what her views are on the Home Office. She doesn't know much about foreign affairs."

A spokesman for Mr Clarke, himself a candidate for the party's leadership in the past, said he would not be commenting on the clip, which was filmed in Sky's studios.

Mr Clarke, who was speaking with another Tory party figure, Sir Malcolm Rifkind, who is mostly off-camera during the conversation, said he did not believe party members would vote for Justice Secretary Michael Gove.

'Ridiculous': Former mayor of London Boris Johnson. Photo: Bloomberg

Mr Gove who was a surprise entrant to the contest last week after withdrawing his support for fellow Brexit campaigner Boris Johnson, effectively ending the former London mayor's hopes.

"I remember being in a discussion about something we should do in something like Syria or Iraq and he was so wild," said Mr Clarke. "With Michael as prime minister we would go to war with at least three countries at once." 

But Mr Clarke said he was happy to see Mr Gove head off a run by Mr Johnson. 

'Wild': Michael Gove, UK justice secretary and leadership contender. Photo: Bloomberg

"He did us all a favour by getting rid of Boris. The idea of Boris as prime minister is ridiculous."

"I don't think either Andrea Leadsome or Boris Johnson are actually in favour of leaving the European Union."

"I don't think they cared very much either way," responds Sir Malcolm.

Leadership vote

Mrs May opened up a commanding lead over her nearest rival, Mrs Leadsom, on Tuesday in the contest to become Britain's next prime minister. In a first ballot of Conservative members of parliament, Mrs May won 165 votes and Mrs Leadsom, a junior energy minister, won 66.

Former defence minister Liam Fox won the fewest votes, 16, and was eliminated from the battle to replace Mr Cameron.

The drawn-out selection process, in which Mr Gove won 48 votes and works and pensions minister Stephen Crabb 34, will ultimately be decided by about 150,000 Conservative party members in September, once MPs have whittled the field down to two candidates.

Clarke under fire

Female members of parliament rounded on Mr Clarke on Twitter over his comments about Mrs May.

"For 'difficult woman' read knows her own mind, un-clubby, no pushover," said opposition Labour lawmaker Liz Kendall, to which Conservative lawmaker Anna Soubry responded: "Let's celebrate 'difficult' women!!'" Scottish Conservative leader Ruth Davidson tweeted: "Amen, sisters."

Mr Clarke, a Conservative lawmaker for 46 years whose former roles include finance minister, said he did not think Mrs Leadsom was actually in favour of leaving the EU.

Mrs Leadsom, who is expected to make the final two with May, said in 2013 she did not think Britain should leave the EU. "It was only three years ago, she has had a Pauline conversion."

"She does have experience in the city... She is not one of the tiny band of lunatics who think we can have a sort of glorious economic future outside the single market. So long as she understands that she's not to deliver on some of the extremely stupid things she's been saying."

If Mrs May or Mrs Leadsom wins, Britain would have its second female prime minister after Mrs Thatcher.

Reuters