Barack Obama will skip a United Nations summit billed as a "once-in-a-generation opportunity" to make progress on global development, sending Hillary Clinton in his place. The US state department announced on Tuesday that Clinton, as secretary of state, would lead America's delegation to the Rio+20 global development summit next week.

She will be joined by the head of the Environmental Protection Agency, Lisa Jackson, and the White House environmental official Nancy Sutley, as well as state department envoys in charge of international climate change negotiations.

The decision, though long expected, will frustrate environmental campaigners who had been lobbying hard for Obama to make an appearance at Rio. The UN secretary general, Ban Ki-moon, had also been pressing hard, urging Obama publicly and in private conversations to commit to the gathering.

Ban, who has been pushing world leaders to engage in the summit, has repeatedly called the gathering, 20 years after the first Earth summit, a milestone opportunity to put the international community on a better path for sustainable development.

George Bush Sr left it to the last minute to decide to attend the original Rio summit, as he was in the middle of his presidential re-election campaign.

UN officials recognised that Obama's presence at an international event such as Rio would also be political poison in a difficult election year. But they had still harbored a glimmer of hope Obama would drop by, injecting more significance into a summit detractors have dismissed as a junket or talking shop for UN bureaucrats.

Expectations for Rio have fallen even further with the slow pace of negotiations. After eight rounds bureaucrats are still haggling over minor points of the draft agreement, called The Future We Want, and have yet to really engage with the issues, according to those close to the talks.

Other leaders, such as David Cameron and Angela Merkel, are also staying away. But Sha Zukang, the UN undersecretary for development, who is overseeing preparations, said there would still be a strong presence of world leaders, including Russia's Vladimir Putin, France's François Hollande and India's Manmohan Singh.

More than 130 heads of government or state are due in Rio next week, compared with 110 at the 1992 gathering. Sha told reporters in Rio that elections and the economic crisis in Europe were a factor in leaders' decisions. "It's their internal problems and a question of each sovereign country to say who will attend the conference. But we are happy if they change their mind and decide to be here," he said.

(By Suzanne Goldenberg, Guardian.co.uk, 13/06/2012)