Keir Starmer supported Andy Burnham in the leadership contest
Jeremy Corbyn's sensational Labour leadership win had more to do with a "yearning" for something different than his own qualities, an MP has said.
Sir Keir Starmer joked that the new Labour leader was "not the Messiah".
Labour had to seize the opportunity of the flood of new supporters Mr Corbyn had attracted to the party, he argued.
But unless it developed "radical" new policies that were not just anti-austerity it would lose the next general election, he warned.
Sir Keir, who was under pressure to mount his own leadership bid after Labour's election defeat in May, despite being elected to Parliament for the first time on the same day, was speaking after Mr Corbyn's first conference speech as leader.
"I don't think it's about Jeremy," he told a fringe meeting in Brighton.
"Jeremy has got a fantastic mandate, he ran a fantastic campaign. He's not the Messiah, he would be the first to say he doesn't have all the answers, and if you touch Jeremy you are not healed.
"All that's happened is that we have opened up a space. Jeremy has opened up a seam, a rich seam of disaffection, disengagement with politics. People want to come back in now and engage. And that's a really good thing."
'Ambitious project'
As an Arsenal fan, he explained, "I reckon I could go to the home end at the Emirates and get the home fans to sing and shout Arsenal, but that's not a good test of leadership.
"What we do from now on is much, much more difficult."
The former director of public prosecutions, who backed Andy Burnham in the leadership contest, said Labour had to win in 2020 but "we don't have that radical, ambitious project and we have to start working on it now."
He said Labour needed to "think past austerity" because it was a "2015 issue" and focus on issues, such as boosting investment and skills, that would matter in the "2020s and 2030s".
Defeated deputy leadership contender Caroline Flint told the meeting, which was jointly organised by Tory think tank Bright Blue and the Institute for Public Policy Research, that the party should not be seduced into mistaking the "echo chamber" of social media for "the reality".
She said Labour needed a stronger message on immigration to win back votes from UKIP and recalled joining thousands of fervent supporters of left-wing Labour leader Michael Foot in 1983 only to be "in tears" on election night when the party lost.
Shadow International Development Secretary Diane Abbott, a close ally of Mr Corbyn, said she was more "optimistic" about his prospects of winning over the voters in the South of England and Scotland that Labour needs to get back to power in 2020.
"There is the potential there to appeal to a very broad range of people," she argued , because he was not the "standard, former special adviser" type and he believed what he said.
