England hold the Ashes after a convincing 3-1 away series win in 2010-11 and Australia's chances of wresting it back do not look bright following their recent 4-0 drubbing in India.
Pietersen will not play in next month's Champions Trophy one day tournament but is expected back for the Ashes series starting in July.
"If Kevin Pietersen's not available for the Ashes I think Australia will win," Australian 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," Warne added of his former Hampshire team-mate.
England completed a 2-0 Test victory over New Zealand this week to signal the kind of form they are in but the leg-spin great warned the Australians, led by Michael Clarke, were quite capable of pulling off a surprise.
"England are definitely favourites," said Warne, who retired from international cricket in 2007 with more than 1000 wickets.
"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."