Jacques Villeneuve has accused Michael Schumacher of a series of dirty tricks and lying to fans and fellow drivers.
Villeneuve and Schumacher have long had a frosty relationship
The ex-champion said Schumacher would "never" attain the hero status of other great drivers such as Ayrton Senna and thought he was "bigger than the sport".
"Michael isn't a great champion because he's played too many dirty tricks and isn't a great human being," Villeneuve said in a scathing personal attack.
Villeneuve claimed Schumacher would be quickly forgotten when he retired.
The Canadian has had a cool relationship with Schumacher since the German tried to drive his rival off the track while they were disputing the world championship in 1997.
That collision, in which Schumacher came off the worse, was just one a series of incidents highlighted by Villeneuve as examples of the seven-time champion's lack of "class" and "integrity".
In an interview to be published next week in F1 Racing magazine, Villeneuve said: "I think the problem is that you don't ever see his true personality.
He just stared in our eyes and lied
Jacques Villeneuve on Michael Schumacher
"He's a racer - but a pure racer, nothing but a racer and, because of that, I think the day he hangs up his helmet people will just forget him.
"Senna, by contrast, will never be forgotten. Some of that is the James Dean factor, of course, because he was killed in action at a young age, but not all of it.
"I don't even think Michael will live on in people's memories as strong or as long as [Alain] Prost has - certainly not as strong or as long as [Nigel] Mansell has.
"Those people attained a hero status that Michael never has and never will."
Villeneve said Schumacher displayed his true character in the controversial incident in qualifying at Monaco this year, when he deliberately parked his car to prevent title rival Fernando Alonso beating his qualifying time.
"Senna played dirty tricks too but he did it with more class, more integrity.
"When he took Prost out [in the Japanese Grand Prix] at Suzuka in 1990, he said he was going to do it before the race," said Villeneuve, who is expected to retire from F1 following BMW Sauber's decision to drop him last month.
"So, unlike Michael, who ridiculously insisted he was innocent at Monaco this year, Senna said, 'Yes, I did it. But I told you before the race that I was going to do it.'
Villeneuve beat Schumacher to the 1997 world title
"That's very different from what Michael did at Monaco and Jerez [in 1997] and Adelaide [in 1994 where he collided controversially with Damon Hill].
"Senna wasn't lying to the fans. Michael was. And the sad thing is that, of course, the fans accept it - they swear black is white, in fact - just so that they can go on respecting the sport they love. And Michael takes advantage of that loyalty."
Villeneuve was also infuriated by Schumacher's behaviour when the F1 drivers discussed the Monaco incident at a Grand Prix Drivers' Association meeting at the British Grand Prix.
"He lies not only to fans but to his fellow drivers, too," said Villeneuve.
"At the GPDA meeting at Silverstone he lied to us and he didn't even have the decency to appear embarrassed about it. He just stared in our eyes and lied.
"And we all knew we were being lied to but very few of us bothered to say anything.
"It's quite sad, really - because the reason Michael did what he did is that he thinks he's better than the rest of us.
"He thinks he's bigger than the sport, too, but he isn't. And when he retires, and no-one really remembers him, that will become clear."