The head of the UN climate change panel (IPCC), Rajendra Pachauri, has resigned amid sexual harassment allegations.
In a letter to UN Secretary General Ban Ki-moon, Mr Pachauri said he was unable to provide strong leadership.
Indian police are investigating a complaint from a 29-year-old woman working in his office in Delhi.
Lawyers for the woman say the harassment included unwanted emails as well as text and phone messages. Mr Pachauri has denied the allegations.
Mr Pachauri, who had chaired the IPCC since 2002 and whose second term was due to end in October this year, denies any wrongdoing and says his email account and mobile phone were hacked.
His office at the Energy and Resources Institute in Delhi (TERI) said he had gone on "long leave" from the organisation.
On Monday, the 74-year-old had pulled out of a high-level IPCC meeting in Kenya starting on Tuesday, because of "issues demanding his attention in India", an IPCC spokesman said.
In his letter to the UN secretary general, Mr Pachauri said his inability to travel to Kenya showed that he might not be able to provide the "strong leadership and dedication" needed by the panel.
"I have therefore taken the decision to step down from my position as chair of the IPCC some months before completion of my term," he wrote.
Analysis: Roger Harrabin BBC Environment Analyst
Dr Pachauri’s resignation is a shock - but it is unlikely to create lasting damage to the IPCC as he was due to retire, and potential replacements are already throwing hats in the ring.
Good luck to them: the IPCC chair is one of the most gruelling and controversial jobs on the international stage.
The chair has to spend much of his life in mid-air, flitting between capitals, whilst suffering relentless attacks from campaigners challenging climate science.
Known to friends as "Patchy", Dr Pachauri's tenure has been controversial. He was installed after the US said the previous chair was too alarmist. They thought Dr Pachauri, as an Indian transport economist, would take a pro-development line.
After immersion in scientific research he too became persuaded that climate change is a real threat, deserving more action from political leaders – and he said so in outspoken terms.
The US then resisted his bid for a second term as chair, but Dr Pachauri was supported by developing countries, who appreciated his willingness to prod the main perpetrators of climate change.
The IPCC has since confirmed that its vice-chairman Ismail El Gizouli will be chairing the four-day meeting in the Kenyan capital which is meant to pave the way for an international agreement in December on curbing carbon emissions.
The move will "ensure that the IPCC's mission to assess climate change continues without interruption," said Achim Steiner, executive director of the UN Environment Programme (UNEP).
In 2007 Mr Pachauri collected the Nobel Peace Prize on behalf of the organisation for its work in the scientific assessment of the risks and causes of climate change. The IPCC shared the award with former US vice-president and environmental campaigner, Al Gore.
In 2010 Mr Pachauri rejected pressure to step down when errors were found in the IPCC's Fourth Assessment Report.