Jeremy Corbyn has just 18-months to prove himself worthy of being prime minister or it will be too late, David Blunkett has warned.
The former Labour home secretary, now Lord Blunkett, called today for "sane, sensible" people of the left to join the party in order to make it electable.
"I think Jeremy has a very short space of time, in politics a short space of time is 18-months to two years, to actually demonstrate he wants to be prime minister, he knows how to be prime minister and if he was people would feel confident in what he said and did," Lord Blunkett said.
Speaking to BBC Radio 4's Today programme, the former cabinet minister said voters were currently"bewildered" by Corbyn's leadership. "I think he has lot to learn," he said.
Following a row between Ken Livingstone and shadow defence minister Kevan Jones, Lord Blunkett also said Livingstone should stand down as co-chair of the party's review of defence policy.
Lord Blunkett said Corbyn, having "moved from dissident oppositionist within 8-weeks to being leader of a major party", had to recognise Labour supporters wanted a general election victory not "a permanent revolution where you become simply a campaign group".
"I want your listeners," he told Today, "if they are interested in politics, and left of centre, to do something counter intuitive at the moment. I want them to join the Labour Party. I want them to join it so their voice of sane, sensible people wanting radial policy can get the Labour Party into a position, with those radical policies, but in a credible situation to win the next general election."
However despite his criticism of Corbyn, Lord Blunkett advised shadow cabinet ministers not to quit the frontbench. "Stay in there, this is not the moment to hive-off bit by bit," he said.
David Cameron is pushing Labour MPs to defy Corbyn and back him in any vote to expand the RAF's bombing campaign against Isis from Iraq into Syria. However the Labour leader is opposed and has indicated he will not allow his MPs a free vote on the issue.
Lord Blunkett, who supports the idea of a wider military campaign, said Corbyn should offer a free vote. And said if he were a shadow cabinet minister who wanted to vote with the prime minister "I would abstain [on the vote] and challenge Jeremy Corbyn to sack them".