Solheim's resignation is effective November 22.
"The Secretary-General is grateful for Mr Solheim's service and recognises that he has been a leading voice in drawing the world's attention to critical environmental challenges, including plastics pollution, climate action, the rights of environment defenders, biodiversity, and environmental security," Dujarric said.
Deputy Executive Director of UNEP Joyce Msuya, from Tanzania, will be appointed as acting executive director while the secretary-general launches a recruitment process, in consultation with member states, to find a successor to Solheim.
In a report, The New York Times said the audit of official travel undertaken by the UN's Office of Internal Oversight Services cited "uneconomical routing of flight itineraries, opting for more expensive airlines, implementation of teleworking arrangements that were outside the existing policy on flexible working arrangements". It also noted a failure to account for absences from the office.
The audit also found that Solheim had spent 79 per cent of his time away from the agency's headquarters in Nairobi and incurred USD 488,519 in travel expenses over a 22-month period. The audit found he had travelled for 529 out of the 668 days audited.
In a statement, Solheim said he has maintained throughout this process that "I have been and remain committed to doing what I believe to be in the best interest of UN Environment and the mission we are here to achieve. For this reason, after deep reflection and in close consultation with the secretary-general, I am stepping down as Executive Director of UN Environment effective Thursday".
In his parting statement, Solheim referred to Prime Minister Narendra Modi's "historic announcement" to phase out single-use plastics and the "incredible partnerships" on nature-based farming in countries like Indonesia and India, and said "we can be immensely proud of our accomplishments and the momentum for action we have created".
Dujarric said Solheim has "led advocacy at the highest levels of government, business and civil society to drive the transformational change needed to make a real difference in the lives of people and promote the cause of environment".
When asked about the secretary-general's comment on Solheim's travel expenses that trigged the audit and led to his resignation and if there would be any further action against Solheim, Dujarric said the secretary-general "is pleased to see that UNEP is committed to implementing the recommendations that are found in the OIOS (Office of Internal Oversight Services) report on the travel office".