During the vote, each of the body's 15 members will be asked to indicate if it "encourages", "discourages" or "has no opinion" on the candidates.

The results are not to be made public, but will be made known to the candidates. It is expected that several of them will drop out of the race due to lack of support.

"Depending on the results of this first vote, a second may be held before the end of the month," the organisation explained.

"Several informal meetings with the candidates are also expected."

There are currently 12 candidates, half of them women, but more could yet throw their hat into the ring.

Among leading contenders are Guterres, former UN High Commissioner for Refugees, Susana Malcorra, the foreign minister of Argentina, Helen Clark, a former prime minister of New Zealand head of the UN Development Programme, and Irina Bokova, a former foreign minister of Bulgaria and director of UNESCO, the UN Educational, Scientific and Cultural Organisation.

Last week Guterres took part in the first public debate between candidates for secretary-general, an initiative unprecedented in the UN's history. During the debate, he said he backed reforming the Security Council and that he would bring in gender parity in his appointments to UN posts.

He also argued that the next secretary-general must be "solid", a "symbol of unity" and that he "must know how to combat, and defeat, political populism, racism and xenophobia."

The final decision on the new secretary-general will be made behind closed doors, with the five permanent members of the Security Council (US, UK, Russia, France and China) having a veto. The choice is then to be submitted to the General Assembly for its approval.

Several states and non-governmental organisations are calling for a woman to be chosen, for the first time. There is also pressure for the figure to be from Eastern Europe, a region that has never produced a UN secretary-general.

The incumbent, Ban Ki-moon, ends his second mandate late this year.