Jan Kubis had succeeded Ghassan Salame, who quit his position in March last year due to stress.
The UN Libya mediator Jan Kubis has resigned less than a year after he took up the role and a month before planned elections in the country.
UN Secretary-General Antonio Guterres accepted Kubis' resignation "with regret," spokesperson Stephane Dujarric said on Tuesday.
When asked when Kubis would leave, Dujarric said: "Mr Kubis has made it clear that he's not slamming the door today."
"The Secretary-General is working on an appropriate replacement. We are fully aware of the electoral calendar and are working as quickly as possible to ensure continuity of leadership," Dujarric said.
The United Nations is informally suggesting veteran British diplomat Nicholas Kay as a replacement, said diplomats, speaking on condition of anonymity.
The 15-member UN Security Council, operating by consensus, must greenlight a new appointment.
Kubis is a former Slovak foreign minister who has also served as the UN special coordinator for Lebanon and the UN special envoy in Afghanistan and Iraq.
READ MORE: Q&A: Why Libya’s elections must be delayed
A UN political forum last year demanded parliamentary and presidential elections on December 24 as part of a roadmap to end Libya's civil war.
The parliamentary election has been delayed to January or February.
However, disputes over the planned vote threaten to derail the UN-backed peace process.
Kubis's predecessor, Ghassan Salame has quit his role in March 2020 due to stress.