Chuka Umunna has urged Labour to unite around its new leader in an apparent offer of reconciliation from the party's modernising wing to leadership favourite Jeremy Corbyn.
Mr Umunna, who quit the leadership race in its early stages, insisted that the party must accept the result and "support our new leader in developing an agenda that can return Labour to office".
The shadow business secretary, often seen as a Blairite, said MPs and others cannot dismiss those who criticise New Labour, such as Mr Corbyn, and said the party should accept its new members and supporters, including the young who may be more supportive of the left winger.
In a speech to the Policy Network think-tank and published in full by the New Statesman, Mr Umunna said: "Solidarity is key which is why we must accept the result of our contest when it comes and support our new leader in developing an agenda that can return Labour to office.
"I do not think we can simply dismiss out of hand those who hold critical views of New Labour.
"Like any government, the New Labour administration made mistakes - it could and should have achieved more, and done more to challenge the right's assumptions about the world.
"In the end, it is not unreasonable to be ambitious for what your party in government can achieve in building greater equality, liberty, democracy and sustainability.
"It is far better we acknowledge, not reject, this ambition for a better world, as we seek to forge a new politics of the common good fit for the future."
Mr Umunna said one of New Labour's "huge weaknesses" was its central control as he called for former Barack Obama adviser and community organiser Arnie Graf to resume his role with the party to harness the power of thousands of new supporters.
He said: "Over half a million people are now members, supporters or affiliated supporters of our party, with hundreds of thousands joining in the last few weeks.
"Some have joined in order to thwart the pursuit of Labour values but many more have joined to further the pursuit of those values, including lots of young people.
"At a time when so many are walking away from centre left parties across the Western world and many young people do not vote let alone join a party, this is surely something to celebrate."
Meanwhile, Mr Corbyn hit out at journalists for "misreporting" comments in which the Islington North MP appeared to suggest the killing of Osama Bin Laden was a tragedy.
He told ITV's News At Ten: "They (the comments) were quite cynically and deliberately misreported by some people.
"The point I made was it was a culmination of a series of awful events all across the region.
"And I said that if they were capable of getting to Osama Bin Laden, then put him on trial."
He added: "It sometimes feels that there is an awful lot of journalists spending an awful lot of time trawling everything I've ever said, everywhere I've ever been."
Mr Corbyn had made the remarks shortly after the 2012 special forces raid on the al Qaida chief's Pakistan compound in which he and four others were shot dead.
In a clip from the Iranian Press TV show The Agenda, Mr Corbyn is heard complaining that there had been "no attempt whatsoever that I can see to arrest him and put him on trial, to go through that process".
He went on: "This was an assassination attempt, and is yet another tragedy, upon a tragedy, upon a tragedy.
"The World Trade Centre was a tragedy, the attack on Afghanistan was a tragedy, the war in Iraq was a tragedy."
The leadership contest is entering its final stages with the winner of the race to succeed Ed Miliband due to be announced on September 12.