Former heavyweight champion Lennox Lewis has warned David Haye against fighting Vitali Klitschko.

Haye lost to Wladimir Klitschko on points in July and it was widely expected that it would be his last bout.

The 30-year-old had planned to retire in October ahead of his 31st birthday but has recently said that he is willing to delay retirement if either Vitali or Wladimir agree to fight him.

However, Lewis is wary of the threat posed by Wladimir's elder brother Vitali after the Ukrainian beat Polish challenger Tomasz Adamek in the tenth round last week.

"At this point, Vitali is untouchable and I cannot see anyone who would be able to [beat him]," Lewis told boxingscene.com.

Haye is reportedly ready to delay his retirement by up to six months if either Klitschko brother agrees to a fight.