Ken Livingstone has told talkRADIO that the changes made to Labour's internal structure by Tony Blair led to the selection of Jared O'Mara as an election candidate.
There have been suggestions that O'Mara, who has now been suspended by Labour, was not even interviewed by the party before he was invited to contest the Sheffield Hallam seat against Nick Clegg in June's snap election.
But Livingstone suggested O'Mara was a last-ditch choice to contest a seat that Labour believed to be unwinnable.
"In the good old days before Tony Blair, a local Labour party got to choose its candidates," Livingstone said.
"People applied, you went to endless meetings, all the local wards, then eventually the general management committee decided who the candidate should be.
"Then Blair introduced this thing where there was approved lists that you had to choose from. But the problem was, they just focused on the winnable seats and nobody assumed we were going to win this seat.
"So it was just a last-minute, desperate thing: 'Find a candidate to put in there, he's can't possibly win, blah blah blah, because he's up against Nick Clegg.'
"And then, to everyone's amazement, he did win."
O'Mara has been suspended from the party in the wake of allegations he'd made misogynistic and homophobic abuse in online posts, as well as an incident in a nightclub in March.
However Livingstone, who has himself been suspended from the Labour Party over allegedly anti-semitic comments, says the process can take a long time.
"Even if he’s expelled from the party, he could hang on as an MP up until the next election which could be over four years away," he told Paul.