Contender for the leadership of the UK’s Conservative Party Andrea Leadsom is facing calls to stand down as the row over her comments on motherhood intensifies.

Allies of the other candidate Theresa May have stepped up their attack on Mrs Leadsom after the Energy Minister endured a barrage of criticism after suggesting that being a mother gave her an advantage over her childless rival Theresa May.

Conservative MP Sarah Wollaston wrote on Twitter that Mrs Leadsom "has repeatedly shown that she lacks judgement and is not the right person to lead the nation. She should now withdraw".

Business minister Anna Soubry called on Mrs Leadsom to quit the leadership contest, saying she was clearly not "PM material", adding it would "do us all a favour including herself".

Foreign Secretary Philip Hammond said that what made someone qualified to be prime minister had nothing to do with whether they had children.

Speaking from the NATO summit in Warsaw, he said: "I think what makes you qualified to be prime minister is having long experience, a clear understanding of the big issues facing this country, and a proven track record of being robust in the face of the many pressures that people at the front line of politics face all the time. That's why I'm backing Theresa May."

He went on to tell say that the Tory party did "not want to witness a slanging match", adding: "They don't want to see back-stabbing. They want a clear, open debate."

According to The Sunday Times, as many as 20 Tory MPs are said to be prepared to leave the party if Mrs Leadsom is elected leader.

A Conservative cabinet source was quoted in the newspaper saying: "If Leadsom wins, I think what are at the moment hushed conversations and tea-room chat will quite quickly turn into something more serious.

"There is certainly a caucus there, probably on our side 10-20 who have mooted the idea in one form or another. But the trigger would have to be the Leadsom-led Conservative party - someone so out of line with the left of the Tory party. We can't go back to the IDS years. We would have to do something about it."

On Saturday, Mrs Leadsom said she was "disgusted" by the way her comments to The Times had been presented and stressed that she did not want the issue of children to be a feature of the campaign.

In an interview with the newspaper, Mrs Leadsom said: "Genuinely I feel that being a mum means you have a very real stake in the future of our country, a tangible stake."

Theresa May, the Home Secretary, has previously spoken about how she and husband Philip were affected by being unable to have children.

Armed forces minister Penny Mordaunt, a supporter of the Energy Minister's leadership bid, said The Times report was an attempt to "smear" Ms Leadsom.

Another supporter, MP Tim Loughton, said: "She's a very genuine person, and she gave an interview to a very experienced journalist at The Times who clearly wanted to lead her down a path.

"Andrea made it absolutely clear that we're not running negative stuff.

"We're not about undermining the other candidates.

"What makes her passionate in politics is her children and her family. Since when has it been a crime to be proud about your children?"

But senior Tory backbencher David Davis said she was not ready to be Prime Minister.

He told the BBC: "(Leadsom) is intelligent and charming and so on, but what the events of the last week have demonstrated is that she has come under a bit of pressure - because leadership contests, as I know to my cost, are somewhat pressurised.

"But they are nothing like as pressurised as being prime minister."

Veteran Tory MP Sir Eric Pickles and a supporter of Ms May said: "She gave a bad interview, her very first interview with a serious journalist, and she fluffed it - she messed it up.

"To make it worse she then accused that very senior journalist of engaging in gutter politics. I don't like to see those kinds of actions in a potential prime minister.

"I think she's got a lot of potential. She should probably have waited for a chance in a future leadership election."

Arron Banks, co-founder of the Leave.EU campaign, said Ms May as Prime Minister "would be the death of Brexit", and suggested that a "new party" could be formed as a result of current political instability.