Jeremy Corbyn has described Keir Starmer's insistence that the pair were never friends as "primary school stuff".
The ex-Labour leader, who has been blocked from running for Labour in Islington North, said the comments made him "sad" and he had always regarded Mr Starmer as a "colleague".
Mr Starmer distanced himself from his predecessor in a recent interview, where he denied he'd ever been friends with Mr Corbyn.
The Labour leader said he hadn't spoken to Mr Corbyn for two and a half years after he was suspended over his response to the damning Equalities and Human Rights Commission (EHRC) report in 2020 into Labour's handling of anti-Semitism complaints within its ranks.
"It makes me sad, actually. I don't know why he says that," Mr Corbyn told LBC.
"I mean, I worked with Starmer. He was in the shadow cabinet. I went to Brussels with him. Yeah, I worked with him, I worked with lots of people."
Asked if he regarded Mr Starmer as a friend, he said: "I regarded him as a colleague. I never regarded him as a friend. I didn't spend time hanging out with him.
"A friend is somebody you go out for a meal with, have a chat, call their house.
"So, why he suddenly announced to the world that I was a friend and then a short time later, now I was not a friend. This is primary school stuff."
Mr Corbyn, who sits as an independent MP, also criticised Labour's recent attack adverts on Rishi Sunak, which he described as "bad news all around".
The party sparked controversy with several graphics released over the Easter weekend targeting the Prime Minister over his record on crime, which included a claim that the PM doesn't think adults who sexually assault children should be locked up.
Former Shadow Chancellor John McDonnell spoke out against the ads, which provoked disquiet among some Labour MPs.
But a meeting of the Parliamentary Labour Party on Monday night passed without any outbursts, with a source saying no MPs asked about the adverts.
Mr Corbyn said: "I think the idea that you say something unbelievably awful about an opponent as a kind of gesture towards them is not very good or very sensible.
"I do not believe in that sort of thing."
Last month, Labour's ruling National Executive Committee backed a motion proposed by Mr Starmer to block Mr Corbyn from standing as a party candidate in the north London seat he has held since 1983.
Mr Corbyn has criticised the move at the time as a "shameful attack on party democracy".
