THERESA May is unfit to be the next Prime Minister because she was on the wrong side of the Brexit debate, her most credible rival has suggested.

Andrea Leadsom said the next Conservative leader, and hence the next Prime Minister, had to be a Leave supporter not someone “reluctantly following the wishes of the people”.

In an interview, the Brexiteer did not refer to the Home Secretary by name, but May, the current favourite, was widely known to be “reluctantly Remain”.

Asked whether the next Tory leader had to be from the Leave camp, Leadsom said: “I certainly think [so] because I’ve been absolutely closely involved and very passionate about the opportunities from Brexit.

“I think it’s very difficult for somebody who doesn’t agree with that, who is reluctantly following the wishes of the people. I think it’s quite hard for them to really see the opportunities.

“I genuinely believe that if we want to make a go of it we need somebody who believes in it.”

She also said key negotiators for the Brexit process should be true believers.

“Obviously in Government you need highly competent people to do the job. But certainly in actually negotiating our future arrangements with the EU, it’s got to be someone who really believes we can do it and a team who believes we can do it.”

Leadsom, a low-profile energy minister before strong performances in TV debates on behalf of Leave, yesterday picked up the support of former work and pensions secretary Iain Duncan Smith.

The arch Eurosceptic praised Leadsom, the MP for since South Northamptonshire since 2010, for her “ability to achieve objectives even against considerable odds”.

He said: “She has warmth, a genuinely human touch and a great sense of humour and her enormous business experience speaks volumes for her ability to handle pressure.

“I believe that Andrea’s strong family background, business experience, compassion, commitment to social justice and dedication will make her a great Prime Minister for the UK.”

May is the frontrunner to replace David Cameron in Downing Street, and yesterday had the support of 103 of the 330 Conservative MPs, including Scottish Secretary David Mundell.

Justice Secretary Michael Gove had 23, work and pensions secretary Stephen Crabb 22, Leadsom 20, and former defence secretary Liam Fox just nine.

However Gove has been struggling to build support since pulling the rug from under Boris Johnson on Thursday by entering the race after repeatedly denying he could ever be PM.

Gove also attacked his former Leave ally as incapable of leading the country, in a move Johnson’s supporters regarded as an act of unforgivable treachery.

His ruthlessness has caused other Tory MPs to reconsider whether they can trust him.

Gove’s problems mean Leadsom is now the champion of many Leave MPs and hotly tipped to be May’s rival after Tory MPs starting whittling down the field on Tuesday.

After a series of elimination votes, the final two candidates will go through to a ballot of party members, with the result due by September 9.

Appreciating how her soft Remain position could count against her in the contest, May tried to pre-empt the issue when she launched her leadership campaign, insisting that “Brexit means Brexit” lest anyone accuse her of backsliding on withdrawal from the EU.

“The country voted to leave the European Union, and it is the duty of the government and of parliament to make sure we do just that,” she said.

May is also potentially vulnerable over immigration, which rose to 330,000 last year during her reign at the Home Office, despite a Tory manifesto pledge to bring it under 100,000.

Business minister Anna Soubry yesterday suggested there ought to be a coronation rather than more blood-letting in the Conservative party and uncertainty for the country.

“It would be best if the candidates among themselves could just back one person and then we can get on with it,” she said.

“This uncertainty — for this to drag on till September — is not great for our country.”

Crabb, previously tipped for high office by Scottish Tory leader Ruth Davidson, yesterday appeared to distance himself from his previous opposition to gay marriage.

The Christian MP said he did not disapprove of same-sex relationships, and only voted against gay marriage because he was concerned about religious freedom.

He said: “I’ve got nothing against people committing to each other whatever their sexuality. That’s a good thing in society. My concern at the time was … about freedom of religion.”

He also said politicians should be allowed to talk about their faith.

“There are moments where it’s a good thing for people in all professions to be a bit more open. We don’t want faith to be something people should be worried to speak about.”