Chuka Umunna has dramatically quit the Labour leadership race, citing "very real concerns and worry about this bid's impact on those close to me".
In a surprise statement, the shadow business secretary blamed the "added level of pressure that comes with being a leadership candidate" and said: "I have not found it to be a comfortable experience.
"One can imagine what running for leader can be like, understand its demands and the attention but nothing compares to actually doing it and the impact on the rest of one's life. Consequently after further reflection I am withdrawing my candidacy."
Sources close to Umunna insisted that his withdrawal was not due to any negative story which he expects to appear in the media.
It is understood that he was uncomfortable with the increased level of scrutiny which had been directed at him and members of his family since he declared his intention to stand for the leadership.
His team were confident that he would secure the 35 nominations from fellow MPs required to mount a leadership bid, but he decided that it was not the right time for him to press ahead.
He is not at this stage endorsing any other candidate for the leadership.
Umunna, who said he hoped to remain a member of the shadow cabinet, apologised to those who supported his bid and said he had always harboured doubts about whether he was ready to step up to the biggest role.
"I apologise to all those who have kindly supported and encouraged me to do this and for disappointing them. I know this will come as a surprise to many but I had always wondered whether it was all too soon for me to launch this leadership bid - I fear it was.
"Most importantly, I continued to have very real concerns and worry about this bid's impact on those close to me.
"I intend to carry on playing my full role as a proud member of our shadow cabinet taking on the Tories.
"I also hope to play a leading role in Labour's campaign to keep the UK in the EU during the forthcoming referendum, which is absolutely crucial. Most importantly, I will as ever continue to serve the area I know and love - the Streatham parliamentary constituency."
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At the weekend the politician was pictured arriving at TV studios for an interview hand in hand with his girlfriend, who had previously remained out of the public eye.
He said he had decided before the campaign started that he would pitch for the leadership if Mr Miliband failed to steer the party back into power.
"I dearly hoped Labour would win the election and it was a decision I would not have to implement. I also thought I understood the scrutiny and attention a leadership contest would bring."
The unexpected withdrawal means there are now four declared candidates: shadow health secretary Andy Burnham; shadow home secretary Yvette Cooper, shadow international development secretary Mary Creagh; and shadow health minister Liz Kendall.
Under a timetable drawn up by the party earlier this week, nominations for leader close on June 15. Members and supporters who sign up by August 12 will be entitled to vote and the result will be announced on September 12.
Shadow education secretary Tristram Hunt said he was "interested in the leadership" of the party while on BBC Question Time but stopped short of declaring his candidacy.
Some Labour figures questioned whether Dan Jarvis - the Para-turned-politician who ruled himself out of the contest to concentrate on family life - could re-enter the frame.
Bassetlaw MP John Mann wrote on Twitter of the Barnsley Central MP: "With Chuka withdrawing - a credible option according to some in my area - can Dan Jarvis not be persuaded to reconsider?"
He suggested: "Make it possible for him to do fatherhood and leadership and move party HQ to Yorkshire."
Here is the statement by Umunna on withdrawing from the Labour leadership election in full:
"Shortly before the election campaign, I made the decision, in the event that Labour was defeated and a new Leader was to be elected, to stand for the leadership of the party if there was a desire in the party for me to do so.
"I dearly hoped Labour would win the election and it was a decision I would not have to implement.
"I also thought I understood the scrutiny and attention a leadership contest would bring.
"As a member of the Shadow Cabinet, I am used to a level of attention which is part and parcel of the job. I witnessed the 2010 leadership election process close up and thought I would be comfortable with what it involved.
"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.
"I have not found it to be a comfortable experience.
"One can imagine what running for leader can be like, understand its demands and the attention but nothing compares to actually doing it and the impact on the rest of one's life.
"Consequently after further reflection I am withdrawing my candidacy.
"I apologise to all those who have kindly supported and encouraged me to do this and for disappointing them. I know this will come as I surprise to many but I had always wondered whether it was all too soon for me to launch this leadership bid - I fear it was.
"Most importantly, I continued to have very real concerns and worry about this bid's impact on those close to me.
"I intend to carry on playing my full role as a proud member of our Shadow Cabinet taking on the Tories. I also hope to play a leading role in Labour's campaign to keep the UK in the EU during the forthcoming referendum which is absolutely crucial. Most importantly, I will as ever continue to serve the area I know and love - the Streatham parliamentary constituency."