David Miliband has warned the Labour Party has been sent "back to the classroom for the second time in five years" after their crushing defeat in the general election.
The former foreign secretary, currently president of the International Rescue Committee, also made comments that will fuel speculation about a return to British politics, boasting about his role in winning previous elections with Labour.
He said: "I was in the back room in the early 1990s when Labour in the UK figured out how to win elections rather than lose them."
In a lecture at the Harvard Kennedy School, Miliband blasted of David Cameron's foreign policy, saying that Britain is now politically "in retreat".
Ed Miliband's brother, David, has sent his commiserations to Labour after the general election loss.
Ed beat his older brother to the Labour leadership in 2010.
Heart goes out to great colleagues who lost seats, Labour teams who worked so hard and of course to Ed. (1/2)
Deep and honest thinking required to rebuild progressive politics. (2/2)
David Miliband has revealed who he's voting for in the General Election ... and unsurprisingly it's Labour.
Sharing a picture of his postal ballot envelope on Twitter, Ed Miliband's brother and one-time political opponent said he was "proud" to have voted for the party.
David Miliband has backed his brother Ed saying he would make a good prime minister - and hinted a return to politics may be on the cards.
The former foreign secretary, who lost out in the party's leadership contest to Ed, said his brother has "the clarity, the vision, the determination" to lead the country.
In an interview with the Financial Times, he said his current job as head of NGO International Rescue Committee could be useful for a political career in the future.
He told the paper: "Tony Blair and John Major have said they wish they'd done their post premiership jobs before they became prime minister."
Ed Miliband has endured an intense bout of speculation about his future recently, with deep unrest among the ranks of backbench MPs and claims that some of his top team are plotting against him.
Last month, Tony Blair said he thought Mr Miliband is ''robust enough'' to deal with the swirling unrest over his leadership, and offered his ''full support''.
Ed Miliband has revealed that he and brother David are working on their relationship after they went head-to-head for the Labour leadership.Read the full story ›
Former Labour MP David Miliband has tweeted saying it is "unbelievable" that a Mail on Sunday journalist attended a private memorial for his uncle Harry.
Woken up in San Francisco to texts about my uncle Harry. Unbelievable. (1/2)
Glad there has been an apology. He was a lovely man. Thanks for messages here this week. (2/2)
The paper has apologised to his brother and Labour leader Ed Miliband.
Read the Mail on Sunday apology here.
David Miliband, who triggered the South Shields contest when he decided to quit as an MP, congratulated Labour's Emma Lewell-Buck, who won the seat with a reduced majority of 6,505.
TV comic Lee Nelson intends to stand for Parliament in the by-election for David Miliband's former seat of South Shields.
The character, created by funnyman Simon Brodkin and who has his own BBC3 series, intends to contest the election on 2nd May after handing in his nomination papers this morning.
Earlier this month Brodkin escaped prosecution following his arrest in March for sneaking on to the pitch and training with the Manchester City team prior to a Premier League match.
Nelson, standing for Lee Nelson's Well Good Party read out a speech on the steps of the Town Hall in South Shields, delivering a "17-point manifesto for a Well Great Britain" with a series of eye-opening policies, including issuing each child with lottery scratch cards at birth.
He added: "David Cameron, George Osborne, Boris Johnson - I wasn't educated at Eton or Oxford. In fact, I wasn't educated at all. It's time to give the posh the push. And let Lee be your leader for a well great Britain."
The by-election triggered by former foreign secretary David Miliband's departure from the Commons is set to be held on May 2.
Labour sources said Mr Miliband would formally vacate the South Shields seat on Monday, and the writ would then be moved for the contest.
Emma Lewell-Buck was selected as the party's candidate for the safe seat earlier this week.
Paolo Di Canio again refused to publicly explain his political views after suffering defeat on his debut as Sunderland manager, but insisted "as a person you don't change".
He could not prevent Sunderland's winless run extending to nine games as Chelsea won 2-1 at Stamford Bridge and declined to comment directly on his views in the post-match media conference.
As a person you don't change, but you become an adult, you become a manager. You can also handle your nature because you know now you're not a footballer, now you have responsibility for many others.
Obviously your nature never changes, but you can lead, you can guide because you know that you have to be careful sometimes when you do something. I don't think you are the same person than 20 years ago, 10 years ago.
We all change. We maintain the principles of when we were growing up, but we change a bit as a man, now as a manager.