Shadow business secretary Chuka Umunna has withdrawn from the Labour party leadership contest, just days after throwing his hat into the ring.
The MP, 36, said in a statement that he was not comfortable with the level of pressure and scrutiny that came with being a leadership candidate.
Mr Umunna also cited concerns about the impact of the bid on his family.
BBC political editor Nick Robinson said the MP's team insisted no "unwelcome press story" lay behind the move.
But Mr Umunna had been shocked by the level of scrutiny of himself, his mother, sister and girlfriend, he said.
A rising star of the party, Mr Umunna was first elected to Parliament in 2010 before joining the shadow cabinet.
He confirmed his candidacy for the Labour leadership via a post on his Facebook page on 12 May, saying the party under Ed Miliband had failed to appeal to enough voters in England.
But on Friday he said he had not found the leadership bid to be "a comfortable experience".
Analysis, by BBC political editor Nick Robinson
I understand that Chuka Umunna has told friends that "this is not right for me or people close to me. I'm human, the rest of my life is more important to me than politics."
The shadow business secretary has been distressed by the degree of scrutiny he, his family and his girlfriend have come under since he declared his intention to stand for the labour leadership.
I am told that this includes reporters waiting outside the houses of relatives late at night.
Friends of Mr Umunna deny that he is nervous of not getting the support of enough MPs to run for the leadership; they point out that he already has half the necessary names and that there are some weeks to go.
The shadow business secretary now wants to play a leading role in arguing that Britain should stay in the EU and does not rule out seeking the leadership in the future.
He said he thought he had understood "the scrutiny and attention a leadership contest would bring", but added: "However, since the night of our defeat last week I have been subject to the added level of pressure that comes with being a leadership candidate."
The shadow business secretary said he had wondered if it was too soon for him to stand for the leadership, and "I fear it was."
He also cited concerns about the impact on "the rest of one's life" and those close to him.
"Consequently, after further reflection, I am withdrawing my candidacy," he said.
Mr Umunna said he would continue to serve as shadow business secretary and hoped to "play a leading role" in Labour's campaign to keep the UK in the EU during the forthcoming in/out referendum.
Commenting on the news, Labour former culture secretary Ben Bradshaw - who is expected to run for deputy leader - said Mr Umunna was a "terrible loss" to the field.
He told the BBC's Daily Politics the Streatham MP had been the candidate the Conservative Party "feared the most".
"And if it's the result of pressure on an extended family or an extended group of friends, I think that's a very sad reflection on our political culture," he said.
Reaction, by BBC Newsnight political editor Allegra Stratton
Chuka Umunna feels "liberated", he told me, by his decision to quit the Labour leadership race.
He hopes to be given a good role (shadow chancellor, I wonder?) by whoever becomes Labour leader. And I expect he'll become very high profile in the campaign to keep Britain in Europe, which is increasingly looking like it will be next May.
He'll endorse a candidate in the next four weeks before nominations close. It will be very interesting to see who he piles in behind.
Former Labour leader Lord Kinnock said he thought Mr Umunna had made the right decision.
He said: "If he felt in his soul that he wasn't prepared to subject himself - and more importantly his family - to the kind of attention which is fairly typical, sadly, these days, he has done absolutely the right thing.
"There is no point at all in inflicting avoidable, unnecessary misery on those you love most."
Leadership contender Mary Creagh said she had "huge respect" for Mr Umunna, telling BBC Radio 4's World at One programme it took "courage" to withdraw from the leadership race.
Mr Umunna was "a big beast" and would play a "big role" in the future of the Labour Party, she added.
Yvette Cooper, Andy Burnham, Liz Kendall and Mary Creagh have already thrown their hats into the ring for Labour's top job while a number of other potential candidates, including Dan Jarvis, Alan Johnson and David Lammy, have ruled themselves out.
Candidates must secure nominations from 34 colleagues - 15% of the party's MPs - by 15 June to make it on to ballot papers, which will be sent to members in August ahead of the leader's election a month later.