Prime Minister David Cameron will not be going to the Rio+20 Summit in Brazil next year despite his pledge to lead the "greenest ever government".
The meeting will mark 20 years since the seminal Earth Summit of 1992, and is regarded as a chance for leaders to put humanity on a sustainable track.
But the June date clashes with the Queen's Diamond Jubilee.
The clash raises the question of whether other Commonwealth heads of government will also stay away.
Protocol would be likely to demand the presence of the 54 countries' leaders in London.
The Jubilee celebrations run 2-5 June, and the Earth Summit 4-6 June; so leaders could theoretically pay their respects in London before heading to Rio for the summit's final day.
But Mr Cameron will not, a Downing Street spokeswoman told BBC News.
Instead, Environment Secretary Caroline Spelman will lead the UK delegation.
"It's a shame Mr Cameron is not going," said Felix Dodds, director of Stakeholder Forum, which is playing a leading role in preparations for the summit.
"It would be important for the British government to show leadership at the highest level in Rio, and we'd hope he'd show leadership by coming to the final preparatory conference the week before."
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