Nelson Mandela said yesterday he was "pained" by Tony Blair waging war on Iraq.
In an astonishing attack on the Premier and President Bush, he said in London: "To see young political leaders of the developed world act in ways that undermine some of the noblest attempts of humanity to deal with historical legacies, pains me greatly and makes me worry immensely about our future."
The 84-year-old former South African president condemned the two leaders for riding roughshod over international law by acting alone.
In a bitter swipe, he added: "We have a common obligation to care for each other, particularly in conditions that foster behaviour to the contrary."
No 10 dismissed the attack, saying: "We have the highest regard for Mr Mandela but on this occasion we do not agree with him."
Mr Mandela hit out as Mr Blair faced a mounting crisis over the allies' failure to find weapons of mass destruction in Iraq, whose claimed existence was given as a reason for war.
Giving the annual Red Cross Humanity Lecture before a 700-strong audience which included Home Secretary David Blunkett, he said: "We have found ourselves compelled to speak out strongly against the rise of unilateralism in world affairs.
"We publicly and in private expressed our sharp differences with Prime Minister Blair and President Bush. The differences we have, particularly in the war against Iraq, are not simply of political difference.
"In a world still so grossly unequal our hope for orderly co-existence lies in global co-operation and an uncompromising multilateral approach to problems and challenges.
"The Geneva Convention and its successors tell us more powerfully than all treaties of the strength of international consensus."