The Islington North MP’s nearest rival, Yvette Cooper, also slammed the left-winger’s policies as “not credible” as polling suggests Mr Corbyn is now the frontrunner in the race to replace Ed Miliband.
On a visit to Scotland yesterday Mr Corbyn dimissed his critics as “nervous about democracy”.
Mr Straw’s remarks followed a warning by former prime minister, Tony Blair, that Labour would face electoral annihilation under the anti-nuclear candidate.
“The person who is most scared of Jeremy Corbyn becoming leader is Jeremy Corbyn,” Mr Straw said yesterday.
He added: “The fundamental problem here is I don’t think anybody remotely believes that Jeremy Corbyn could win power.”
He called Mr Corbyn’s policies an “anti-power manifesto” and warned: “People must understand this is a vote for perpetual opposition.”
And in a further sign of concern that Mr Corbyn – who initially struggled to find a place on the ballot – could emerge victorious, Ms Cooper weighed in to attack her rival’s polices.
She dismissed his plan to effectively print money to fund public investment as “really bad economics” and compared those who “bought into” Mr Corbyn’s pitch to those whose yearning for “something subversive” had led them to switch to the SNP and Ukip.
“We have to look the 21st century in the eye, face up to the future,” she said.
“Not the old answers of the past. They won’t change the world, they will keep us out of power and stop us changing the world.”
Mr Blair warned in an open letter that the prospect of Mr Corbyn winning the leadership won’t be a defeat like 1983 or 2015 at the next election.
“It will mean rout, possibly annihilation,” he said.
But speaking in Aberdeen yesterday where he kicked off a speaking tour of Scottish cities, Mr Corbyn insisted he has put forward a “serious set of economic proposals” which could still be developed amid mounting criticism from opponents.
And he rejected the “personal attacks” which have been launched by opponents in recent days.
“I feel that some of those people that resort to personal abuse, name-calling and all that are probably a bit nervous about the power of democracy,” he said.
He also slammed the UK’s role in Iraq under Mr Blair, which he opposed at the time.
“We are paying the price of Iraq. The Labour party has paid the price of Iraq. I’m determined we will learn that lesson and we will not go to war on behalf of whatever capricious US president happens to be in office at that time, We don’t go to war on the basis of a lie, we won’t go to war knowing that one war leads to another to another to another.”
More than 600,000 people have registered to vote in the leadership election which Mr Corbyn described as a “big mandate” which would allow the new leader to develop policy changes.