Gordon Brown has taken a thinly-veiled swipe at Jeremy Corbyn as he warned the Labour Party needed to be "credible".
The former Prime Minister warned "you cannot deliver principles without having power" in a coded attack on the Labour leader, who he did not mention by name.
Giving a speech on child poverty attacking George Osborne's cuts to tax credits, the former MP underlined how vital "hope" is to addressing major battles political parties face - but that hope cannot be "pie-in-the-sky".
Labour critics fear Mr Corbyn's support for policies such as ditching Trident and reversing austerity, already causing divisions in the party, will serve to hand the Tories another election victory.
His intervention, which echoes comments he made before Mr Corbyn's leadership election triumph, will hearten moderate Labour MPs disillusioned by the party's direction.
His argument can be read as a defence of the "third way" of the Blair-Brown years and three general election victories, and a riposte to critics now more happy Labour has shifted to the left.
The former Chancellor, who still holds sway with the left, said it was "obvious and normal" that a party would "return to first principles" after two election defeats, and that it will "want to emphasise that power is not for its own sake, power has to be based on pursuing principle".
But he added: "They will want also to think of this: that while you cannot deliver policies without principles you cannot deliver principles without having power.
"You have quickly to move to a stage where emphasising your principles, you build a programme then call for popular support.
"Making the desirable possible requires us to make the desirable popular, electable, credible and something that people want to hold on to.”
He added: "It's about in the end hope. Whether you can give people hope something can change in the future.
"Hope cannot be a wing and a prayer. It cannot just be pie-in-the-sky.
"It cannot be wishful thinking that perhaps some day we might be able to do something about child poverty.
"It cannot be sunny but baseless optimism, that we can do something about it, but we don't know what.
"Hope - and this is true of political parties as well as pressure groups - must be based on the realistic expectation that we can actually change things and do something about the problems that we face."