Bush said he would send Secretary of State Colin Powell to lead the U.S. delegation. South Africa has yet to confirm the number of world leaders attending but original expectations were around 100. More than 40,000 delegates are expected.

Environmentalists said the widely expected announcement sealed Bush's reputation as an environmental laggard after he pulled the United States out of the Kyoto pact meant to limit emissions of greenhouse gases mainly caused by fossil fuels.

And in the United States, opposition Democrats cast Bush's planned absence as a retreat from leadership which could backfire as he tries to maintain an anti-terrorism coalition and seek support to oust Iraqi President Saddam Hussein.

"I think it's a mistake that he will regret," said Stephen Sawyer, climate policy adviser at environmental group Greenpeace. "I think that it will be seen as just another example of U.S. intransigence and unilateralism."

He said Washington had been a global leader on environmental issues until the 1980s. The summit aims to find ways to halve the number of people who live in poverty by 2015 without damaging the environment.

LURE OF TEXAS

The World Wide Fund for Nature (WWF) also regretted Bush's absence and said U.S. voters might punish him in November's congressional elections for staying home at his Texas ranch.

But many U.S. Republicans see the summit as promoting a leftist agenda including increased aid for developing nations. Bush pulled out of Kyoto, arguing it was too costly to implement and was unfair because it excluded developing nations.

"WWF thinks it is extremely disappointing that the largest superpower...withdraws from its responsibility to take sustainable development seriously," said Stephan Singer, the WWF's climate change expert.

"We hope Americans will give their response to this in the next elections," he added.

Bush said the U.S. delegation would make proposals to help the main priorities of development - clean water, modern energy, good health, and productive agriculture. Bush agreed to raise U.S. aid levels at a summit in Mexico in May.

Nitin Desai, the Secretary-General of the summit, played down Bush's absence.

"What really matters is how effectively the countries engage, and the United States has fully engaged in the summit," he said. "There was no surprise that Bush was not coming because he never said he was coming."

In Rome, political sources said Italian Prime Minister Silvio Berlusconi would also not attend because he was pressed for time in early September. His decision also prompted criticism from environmentalists and the opposition.

"At a time when public attention and concern is focused on environmental issues it is unforgivable that the Italian government will not be represented at the highest level in Johannesburg," opposition spokesman Dario Franceschini said.

In Brussels, a spokesman for the European Commission said: "We are not focused on the level of participation. The aim of Johannesburg is to get a concrete result."

France and Britain, which had trumpeted British Prime Minister Tony Blair's early announcement that he would attend as a sign of leadership, declined comment on Bush's decision.

(With extra reporting by Toby Reynolds, Johannesburg, and Brussels, Rome and London bureaux.).