London - Shane Warne believes Kevin Pietersen is so vital to England's chances that Australia will regain the Ashes if the star batsman is unfit.
Pietersen is currently recovering from a knee injury and batted in the nets alongside his England team-mates at Lord's on Thursday as they prepared for a one-day series with New Zealand.
In Pietersen's absence, England this week wrapped up a 2-0 series whitewash over New Zealand in their final Test campaign before they begin the defence of the Ashes at Nottingham's Trent Bridge ground in July.
Indications are that South Africa-born Pietersen, who played under Australia great Warne's captaincy at south coast county Hampshire, will be fit
But Warne, arguably cricket's greatest leg-spinner, said England would surrender the Ashes without him.
"If Kevin Pietersen's not available for the Ashes I think Australia will win," Warne told Sky Sports News.
"I think he's that big a player for the England team.
"Everyone feeds off him. He's such a destructive player, he can come out and get some low scores, but he's such a destructive player," added Warne, who fielding at slip, dropped Pietersen during the course of an innings of 158 at The Oval that set the seal on England regaining the Ashes in 2005.
"If Kevin Pietersen plays then I still think Australia can come out and surprise England but I think if he's unavailable for the Ashes, which we don't want, we want him to be playing, everybody does as a cricket lover, if he's not playing for England then Australia will win."
"England are definitely favourites," Warne added.
"They've been playing some excellent cricket, and they're well led by Alastair Cook.
"But I really believe Australia have a squad, if they pick the right players and they perform then, I think the people who are out there writing them off could really be surprised."