Labour needs "restless rethinking" of its purpose and its policies if it is to return to power, former leadership candidate David Miliband has said.

In his most high-profile foray into the political frontline since being defeated by his brother Ed in the 2010 poll, the former foreign secretary set out a seven-point plan for the party.

He said that Labour must admit "loud and clear" where it got things wrong in power, but - in what may be seen as a defence of New Labour against his brother's criticisms - he insisted that the party must assert that the gains made between 1997 and 2010 "far outstripped the mistakes".

Mr Miliband was careful to praise his younger brother's leadership, but his decision to set out his own thoughts on Labour's future direction will inevitably spark speculation that he has not ruled out a return to the party's top ranks.

His intervention, in an essay in the New Statesman, came as Labour's former chancellor Alistair Darling told the same magazine that the party needs to present its policies "in a sharper way".

Mr Darling said: "In politics if you make an assertion that something needs to change I think you have to have an example of how you do it. In relation to growth, I think that's absolutely critical. Do we have to do more to present this in a sharper way? Of course we do."

Mr Darling said he would like to see David Miliband in the shadow cabinet, though he accepted that he was right to fear comparisons with his younger brother.

"I would like him back on the front bench. For his knowledge, and his judgment," said the former chancellor. "When I've seen him on various programmes talking about foreign affairs, he talks with authority. I understand his reluctance. There's always comparisons. He is probably right to take a rain-check. Certainly, he would be a gain."

David Miliband told the New Statesman that Ed should be given credit for preventing disunity in the Labour ranks since its disastrous 2010 general election defeat. And he said his brother had shown he understands the need for a policy rethink and had spoken "powerfully and correctly" about welfare

But he warned that there were elements within Labour who wanted to respond to defeat by retreating to "big state" social democracy. And he said the party had "a lot to be concerned about" in terms of its prospects of electoral victory in 2015, when Conservatives will be boosted by their financial advantages and boundary changes which will favour them.