David Miliband has condemned a "broken" political system which left parties under the control of "small groups".
The former foreign secretary lost out to his younger brother Ed in the fight for the Labour leadership, with the trade union vote crucial in denying him victory.
Perhaps mindful of the bruising Labour leadership contest, Mr Miliband praised the "exemplary" open primary system which had resulted in France's Francois Hollande being nominated as the socialist party's presidential candidate.
In an essay in the New Statesman, Mr Miliband said politics was "not merely seen as broken but actually broken".
He said: "This is not just about corruption scandals. It is also about political systems that, for too many people, have lost their capacity to engage and include.
"Party systems are controlled by small groups (President Francois Hollande's exemplary election by open primary excepted). Big money creates a sense of popular disempowerment.
"Parliaments are gridlocked and enfeebled. For parties of the left, which take pride in the power of politics to check the abuse of power in markets, these are not peripheral concerns. They are the basis for rethinking."
Under Ed Miliband, Labour has begun efforts to change the link between the party and trade unions, with an aim of individual union members to be given the choice of opting in to join the party rather than being automatically affiliated.
In the essay examining the future for centre-left politics, the elder Miliband - who quit Westminster in a surprise move earlier this year to take up a position with New York-based International Rescue Committee - also stressed the need for "fiscal prudence".
"While fiscal stimulus may be a short-term remedy for low demand, fiscal prudence is a medium-term necessity," he said.
Mr Miliband said the welfare system "needed defence in the face of unfair attacks" but "it also needs reform".
He also acknowledged the failures of his party in office, saying by 2010 Labour was "too hands-on with the state and too hands-off with the market".
Mr Miliband insisted Labour could win the next election, but only by building and maintaining a coalition of support.
"Assembling a winning coalition is one thing; holding it is harder. That makes the achievement of the Obama campaign quite remarkable.
"He was helped by his opponent, but he made his own luck through a drive street by street and voter by voter to maintain his winning coalition. Presidential elections are different from parliamentary systems, but there is read-across nonetheless.
"Politics is more open than for many years - and there is a lot to fight for. I will be fighting a different set of battles, but I leave Britain confident that Labour can win."
Conservative Party chairman Grant Shapps said: "Even David Miliband thinks Labour must change.
"By opposing every measure we've taken to get the deficit down and fix the welfare system, Ed Miliband has shown that he's too weak to take the difficult decisions needed to turn Britain around.
"Instead all Labour offer is more borrowing, more spending, and more debt - the same old Labour policy that got us into this mess in the first place.
"And it's hardworking people who would pay the price with higher mortgage rates and higher bills."