Gordon Brown has praised Jeremy Corbyn for the way he has articulated public anger while setting out his vision for a fairer society.
The former prime minister and Labour leader described Mr Corbyn as a “phenomenon” who had restored people’s faith in the party’s principles.
Mr Brown said the challenge he now faced was to show he could produce a programme for government that was “credible and therefore electable”.
“Jeremy is a phenomenon. He has cut through because he expresses people’s anger at what has happened - the discontent,” he told the BBC Radio 4 programme.
“When he attacks Universal Credit he is speaking for many people. When he says the health service is underfunded he is speaking for many.
“What he is saying on these things is absolutely right.”
Mr Brown acknowledged that during his time in office as chancellor and later prime minister, he and Mr Corbyn rarely saw eye-to-eye, with the current Labour leader voting against the government more than 500 times.
He pointed out, however, that Mr Corbyn had since adopted the New Labour slogan “For the many, not the few”, while his shadow chancellor John McDonnell had backed Mr Brown’s decision in office to make the Bank of England independent.
“Jeremy has articulated a view of a fairer society,” Mr Brown said.
“You have got to convert this sense that you have restored people’s faith in your principles to a plan for the future that is credible and therefore electable and a programme that is popular
“They are now with the responsibility - and I hope that they can discharge it - of showing that they can produce a programme that is costed and which is popular and which is both radical and progressive.
“That is the challenge for any left-wing or progressive party.”