Chuka Umunna, has confirmed he will be standing for the Labour leadership, saying he will have no truck with those who say the party faces 10 years out of office.
Umunna travelled to Swindon, where the party failed to win two key seats, to make his announcement on his Facebook page.
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.
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".