Lennox Lewis says Wladimir Klitschko cannot be considered his legitimate successor as the undisputed world heavyweight champion until he possesses the full set of belts.
Klitschko will defend his WBA Super, WBO and IBF titles against Britain's Tyson Fury in Dusseldorf next Saturday having last tasted defeat more than a decade ago.
But Lewis insists the Ukrainian giant, who also holds the IBO belt, needs to match his feat of adding the WBC title to consider himself a true champion across the divisions.
"For that, he needs to have every championship belt. I had them all," Lewis told German daily Die Welt.
British star Lewis retired from boxing in 2004, but his points win over Evander Holyfield in 1999 gave him all four major belts and he says he wants to see Klitschko bow out of professional boxing by achieving the same sweep of all four titles.
"In addition to the IBF and WBA belts you need to have the WBC belt, that is the most important," said the 50-year-old.
"No matter what Wladimir has to arrange, he still has to get this one belt. If he does, he should say goodbye to boxing. It would be the perfect end to a glittering career."
American Deontay Wilder is the current WBC champion and Lewis says time is running out for Klitschko, who turns 40 next April.
"He will reach the point where he can no longer punish himself in training like a 20 or 30-year-old," said Lewis.
"You reach the stage when you can't just jump out bed and flog yourself. Also the risk of injury increases. The biological clock is ticking and it happens to be the toughest rival."
Lewis expects Klitschko to win comfortably against Fury to add to his record of 52 knockouts in 64 wins, while the last of his three defeats was back in 2004.
Fury has won all of his 24 fights to date with 18 knockouts, but despite enjoying a height advantage, Lewis expects him to be unbalanced by the champion.
"He [Klitschko] must have patience," said Lewis. "At some point, the moment will arise where you can use the right."
Wladimir Klitschko vs Tyson Fury, exclusively live on Sky Sports Box Office on November 28, is now available to order via the phone and online.