By Estelle Shirbon

LONDON (Reuters) - Andrea Leadsom, one of two candidates vying to become the next British prime minister, has caused an uproar by suggesting that being a mother means she has a greater stake in the country's future than her childless rival Theresa May.

A little-known junior energy minister until she emerged as one of the most ardent voices in the campaign to leave the European Union, Leadsom is the outsider in the contest to succeed David Cameron as Conservative leader and prime minister.

Cameron, who had campaigned for Britain to stay in the bloc, announced he would quit after the June 23 referendum delivered a vote for Brexit. May, the interior minister who also advocated remaining in the EU, is the favorite to replace him.

"I am sure she will be really sad she doesn't have children so I don't want this to be 'Andrea has children, Theresa hasn't' because I think that would be really horrible," Leadsom, a devout Christian, told the Times newspaper.

"But genuinely I feel being a mum means you have a very real stake in the future of our country, a tangible stake. She possibly has nieces, nephews, lots of people. But I have children who are going to have children who will directly be a part of what happens next."

The comments were denounced by many Conservative lawmakers and May supporters, who used words like "vile", "divisive", "insulting" and "embarrassing" to describe them.

Among them was Foreign Secretary Philip Hammond, who said the leadership contest should not descend into "a slanging match" but focus on substantive issues.

"What makes you qualified to be prime minister is having long experience of the issues facing this country ... That is why I am backing Theresa May," he said.

Leadsom fought back, denouncing the Times article on Twitter as "gutter journalism" and saying in a televised statement: "I want to be crystal clear that everyone has an equal stake in our society and in the future of our country."

The Times, which backs May, stood by its story and released a recording of Leadsom making the comments, which was played on Saturday news bulletins on all the main radio and TV stations.

The Sunday Times, the Times's sister paper, said late on Saturday it had learnt that "as many as 20" Conservative lawmakers were prepared to leave the party if Leadsom became leader - potentially depriving her of a majority in parliament.

"LACK OF JUDGEMENT"

May made no comment on Leadsom's interview, merely tweeting: "Yesterday, I launched my clean campaign pledge and invite @AndreaLeadsom to join me in signing it."

That was a reference to five commitments May made on Friday, including to ensure that campaigning "stays within the acceptable limits of political debate."

In a boost for May, the pro-Conservative Telegraph newspaper declared its support for her on Saturday.

May and Leadsom were selected by Conservative lawmakers from an original field of five candidates. May was backed by 199 lawmakers, while Leadsom received support from 84. The contest now moves to the party's 150,000 grassroots members, who will elect the winner by Sept. 9.

Leadsom's bid, which has enthused some of the more staunchly eurosceptic people on the right of the party, had already been marred by questions over whether she had overstated her professional experience in finance.

Building on her credentials as a Brexit campaigner, Leadsom has said that if elected she would swiftly launch the legal process of extracting Britain from the EU, and that current freedom of movement rules would end.

But her comments on motherhood overshadowed other points she has made since announcing her bid, drawing some strong reaction.

"I am childless. I have nieces and nephews. I believe I - like everyone else - have a very real stake in our country," tweeted Ruth Davidson, the leader of the party's Scottish arm who is backing May.

Lawmaker Alan Duncan, also a May supporter, called Leadsom's comments "vile", while colleague Antoinette Sandbach said Leadsom had shown "a lack of judgment."

Leadsom supporter and junior defense minister Penny Mordaunt defended the controversial comments.

"She was talking about what motivates her personally," Mordaunt told BBC radio, describing the Times article as an attempt to smear Leadsom.

The row comes days after May spoke publicly for the first time, in a newspaper interview, of the sadness she and her husband Philip had experienced after being unable to have children.

(Additional reporting by David Milliken; editing by Gareth Jones and Diane Craft)