HELEN Clark has been named as joint favourite for the United Nations' top job by British bookmakers.

After formally announcing her bid for the Secretary-General position yesterday, Clark was given odds of 7/2, or a 22 per cent chance of winning, by betting company William Hill.

Bulgarian politician and director general of UNESCO, Irina Bokova, also had odds of 7/2.

William Hill spokesman Graham Sharpe said the contest appeared to be wide open, "although the mood music seems to indicate that there is a desire to appoint a woman in the role for the first time".

Irish bookmakers Paddy Power named Clark as the outright favourite, with odds of 3/1, ahead of Irina Bokova on 9/2.

Several current or former country leaders were among the frontrunners, including Angela Merkel, who was given odds of 8/1 by William Hill and 10/1 by Paddy Power.

Former Australian Prime Minister Kevin Rudd, who is considering a bid, was given odds of 25/1 by William Hill.

Prime Minister John Key said yesterday that he believed Clark had an advantage in the contest because she was in a "neutral position".

That meant she was less likely to be vetoed by any of the five permanent members of the United Nations Security Council, he said.