He said he had spoken to half of the Labour candidates who had been defeated in the party’s 80 key seats, as well a large number of parliamentarians and those he described as stakeholders.
He said: “I will be standing for the leadership of the party. I think we can and should be winning in seats like Swindon; north, south, west and east – we can absolutely win as a party.
“Some have in recent days now suggested it is a 10-year project to get the party back into power. I don’t think we should have any truck with that. I think Labour can do it in five years. I want to lead that effort as part of a really big Labour team getting Labour back into office, and building a fairer and more equal society. That is why we joined the Labour party in the first place.”
In his brief Facebook announcement, Umunna said defeated candidates told him the party had lost working-class votes to Ukip and middle-class votes to the Conservatives.
His aides said Swindon was where the Tories launched their manifesto last month. Swindon South was one of Labour’s target seats and Swindon North was a Labour seat between 1997 and 2010. There should be no cap on Labour’s ambitions to take back seats from the Tories, they said.
Umunna did not make an announcement on Sunday, his aides said, because he felt it would be insensitive to do so before he had a chance to speak to defeated candidates, and also was determined not to make the announcement in Westminster. They said politicians needed to get out of the Westminster bubble and engage with the electorate.
He will also be tweeting out interviews with Labour parliamentary candidates from across the country “who ran great campaigns in places we need to win but missed out last week, with their reflections on the result and the lessons we need to learn”.
Earlier, the only other declared Labour candidate, Liz Kendall, said it was “maybe time” that the party had a female leader.
Kendall, the shadow health minister, argued the party needed to “blast out of these old debates about Blairite, Brownite, Old Labour, New Labour and create something new rooted in people’s values and concerns”.
She told BBC Radio 4’s Woman’s Hour: “People might think I’m a bit biased in this, but I think it’s maybe time that Labour had a woman leader. People like Margaret Beckett and Harriet Harman have been the acting leaders of their party, they have blazed a trail and I’d be beyond proud if I was elected as Labour’s leader.”
Asked if the dynamic would be better for Labour if they had a female leader up against David Cameron in the House of Commons, she said: “I don’t know, I think, hopefully people will judge me on the strength of my arguments. There’s a long way to go.”
She said: “We have got to have a long process of talking to people and listening to them to get us back on the right track.”
Kendall vowed that Labour “can win again if we think of the profound changes we need to make as a party and we build something new”.
Andy Burnham, the health secretary, and Yvette Cooper, the shadow home secretary, are also planning to stand, but have not yet declared. Tristam Hunt, the shadow education secretary, is also testing the water to see if he can gather nominations. Cooper and Burnham, the two most senior candidates, are likely to secure the most nominations from MPs, but the election is held on a one-member, one-vote basis of membership and registered supporters, and the result is unpredictable.
Stella Creasey, the Labour MP for Walthamstow, said she was ”open to the question of standing for Labour’s deputy leadership”.
She would be entering a crowded field including Tom Watson, Caroline Flint, and Angela Eagle. In practice, potential candidates are ringing round MPs to see if they are willing to nominate them and so get on the ballot paper.
This article was written by Patrick Wintour Political editor, for theguardian.com on Tuesday 12th May 2015 11.51 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010