The election to replace Ban Ki-moon as Secretary-General of the United Nations (U.N.) reaches a crucial stage today when the world's superpowers make clear their preferred choice.

Ten candidates remain in the race for the job with a decision expected this month as to who will occupy the post from January 1, 2017.

A straw poll by 14 countries takes place today and should narrow the field sharply as we learn preferences of the five veto-wielding permanent members of the Security Council.

The selection is highly political with Russia, who insist the next U.N. leader should come from Eastern Europe, supportive of Bulgaria's Irina Bokova.

Bokova is a Bulgarian politician and the Director-General of UNESCO.

In 2015 she was cited by several media as the front-runner in the race to succeed Ban Ki-moon.

In the last week a late entrance to the race from EU Budget Commissioner, Kristalina Georgieva, has antagonized Moscow.

Georgieva, also a Bulgarian, has the backing of her government and is seen as a preferred candidate by western Europe and in particular, Germany.

The appointment to the role is decided by a series of ballots, where each voting member offers an "encourage", "discourage" or "no opinion" status to each candidate.

Former Portuguese Prime Minister and U.N. High Commissioner, António Guterres, has emerged as the clear leader, but today's round of voting allows vetoes to be enforced by permanent members of the Security Council.

With Moscow openly preferring an Eastern European candidate, it is thought Guterres' run as favorite could swiftly come to an end.

With five voting rounds now complete, the second most "encouraged" candidate is Vuk Jeremic , a 41-year old diplomat who served as Serbia's minister of foreign affairs between 2007 and 2012.

However, it is reported in Serbian media that the pro-Russia stance of the country's politicians means the United States will almost certainly use their veto to end his ambitions.

Half of the candidates are women and in August, incumbent leader Ban Ki-moon expressed that for the first time a woman should take the job.

One candidate, former Prime Minister of New Zealand and current Administrator of the United Nations Development Program, Helen Clark, told CNBC that while she is not currently considered a favorite, today's ballot could soon change that.

"In the end this person is going to be someone that the permanent members, who each carry a veto, can agree on. So you can pile up a lot of votes but not necessarily the votes where you need them," she said.

Clark argued that it is time to lift the profile of secretary-general above that of a diplomat and that her experience as a prime minister puts her as the most qualified person for the job.

"I've been involved across big decisions and when I came to the U.N. development, everything I'd learned about peace, security, economic and environmental issues came in to play, because development is right across that.

"So, I would say in terms of breadth and strength of experience there is no equal to me in this game," she said.

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