By Matt Siegel

SYDNEY (Reuters) - Former Australian Prime Minister Kevin Rudd on Friday withdrew his candidacy to be the next U.N. Secretary-General after current Prime Minister Malcolm Turnbull declared him unfit for the job and refused to back his candidacy.

Conservative leader Turnbull denied that politics played a part in his cabinet's decision not to back the former Labor leader, who had asked the government to nominate him as Australia's candidate.

"Does the government believe, do we believe, do I as prime minister believe that Mr Rudd is well suited for that role? My considered judgment is that he is not," Turnbull told reporters in Canberra.

Rudd, prime minister twice during six years of Labor party strife from 2007 to 2013, is a polarizing figure in Australia due to his rivalry with former Labor prime minister Julia Gillard, with the two toppling each other in party-room coups.

He was always considered a long-shot for the top U.N. job as it usually rotates geographically and Eastern Europe is the only region not to have held it yet.

Rudd's office said he would not continue with his candidacy.

"A nomination by the government would not have granted Mr Rudd a position. It would simply have enabled him to stand alongside the 12 other candidates from across the world, a compete on his merits," the statement from his office said.

"That is now not to be."

In contrast, New Zealand Prime Minister John Key is making a full-throated appeal for former New Zealand premier Helen Clark to take over from Ban Ki-moon.

(Reporting by Matt Siegel; Editing by Paul Tait and Michael Perry)