UN environment chief Erik Solheim has resigned amid a row over his travel expenses.
A recent draft internal audit, obtained by Britain's Guardian newspaper and seen by the BBC, said he had incurred costs of $488,518 (£382,111) while travelling for 529 out of 668 days.
It said this harmed the reputation of UN Environment - a body that highlights green issues and sustainability.
There was "no oversight or accountability" to monitor this travel.
Mr Solheim, a Norwegian former environment minister, says he has paid back the money where "instances of oversight" occurred.
On Tuesday, Mr Solheim himself confirmed his resignation, Norway's NRK broadcaster reported.
A formal UN announcement is expected shortly.
Climate change jet-setter
By Matt McGrath, BBC environment correspondent
Once news leaked about Erik Solheim's flights and expenses, he was on borrowed time at the UN. It wasn't solely because of the large sums involved - there was also an image problem.
At a time when awareness of climate change has never been greater, the sight of the man dedicated to fighting global warming endlessly jet-setting around the world risked reputational damage to the UN.
The auditor's report had also upset some European countries to such an extent that they had threatened to withhold millions of dollars in funding if he continued in post.
While many in UN circles will be glad to see him go, some will miss the energy and greater visibility he brought to a once-lacklustre branch of the organisation.
What did the audit say?
The amount spent on travel by UN Environment, a Kenya-based agency, doubled between 2014 and 2017.
The report was particularly critical of the travel undertaken under Mr Solheim, UN Environment executive director since 2016. A total about $58m was spent in the two years since then.
While all UN staff are expected to complete "mission reports" within two weeks of travel, the audit found that these were often missing.
When they requested reports for 596 trips undertaken by 32 managers and staff, 210 mission reports were not provided, while around 200 others were only completed after the request was made.
The audio was carried out by the UN's Office of Internal Oversight Services (OIOS).
It says Mr Solheim's leave and travel were approved by a member of his office who reported directly to him - in contravention of UN rules.
On one occasion, it says, "he made an eight-hour trip from Washington DC for a weekend in Paris, before taking another flight to New York city."