THE prospects of John Bolton becoming the next US Ambassador to the United Nations have suffered another setback with Colin Powell, his former boss, raising doubts about his fitness for the job.

General Powell told key Republican senators that President Bush’s controversial nominee was a man of suspect temperament who clashed with colleagues.

The general’s intervention comes after lurid accounts of Mr Bolton’s volatile temper and ill-treatment of subordinates raised eleventh-hour doubts about his confirmation.

In a further blow, the former US Ambassador to South Korea has suggested that Mr Bolton misled the Senate Foreign Relations Committee when under oath earlier this month.

Thomas Hubbard said that far from praising Mr Bolton for a 2003 speech on North Korea, as the would-be ambassador had claimed, he had urged him to tone down his antagonistic rhetoric. The upshot is that what looked at the beginning of the week like a straightforward confirmation is in serious danger of falling apart.

On Tuesday the Republican-controlled committee postponed a vote on Mr Bolton’s nomination to permit further investigation of the allegations. Mr Bush had praised Mr Bolton as a “good man”.

Three Republican members have raised public doubts about Mr Bolton and General Powell’s behind-the-scenes role could prove pivotal. He has spoken to two of the wavering Republicans, Lincoln Chafee, from Rhode Island, and Chuck Hagel, of Nebraska.

General Powell’s aides said that he gave the two senators a balanced opinion of Mr Bolton. But the former Secretary of State is known to have clashed repeatedly with the hawkish Mr Bolton, one of his senior officials in the first Bush term.

General Powell notably failed to join seven other former secretaries of state and defence in signing a letter in support of Mr Bolton’s nomination. Lawrence Wilkerson, General Powell’s former chief of staff, told The New York Times that Mr Bolton would make “an abysmal ambassador”.

Mr Chafee, a moderate facing re-election next year in a Democratic state, said he was much less likely to support Mr Bolton in the light of questions about his credibility. George Voinovich, an independent Republican with a maverick streak, is also wavering.

Democrats have said that Mr Bolton’s record of blunt outbursts and confrontational behaviour make him especially ill-suited to the UN job. He has also railed against the institution itself.

White House officials insist that the charges against him — that he is a bully who meddled with intelligence on Cuba’s alleged biological weapons programme and tried to remove officials who disagreed with him — are “trumped up”.

They have also suggested that Democrats would regret it if they successfully blocked the nomination, arguing that only he has sufficient credibility with the Republican-controlled US Congress, which is hostile to the UN, to persuade it that Kofi Annan, the UN Secretary-General, was genuinely reforming the world body.

But Mr Hubbard also accused Mr Bolton of “undiplomatic behaviour”, saying that he failed to attend a dinner the ambassador had arranged with senior South Koreans. On another occasion Mr Bolton angrily hung up on him when Mr Hubbard failed to secure a meeting with the South Korean President-elect.

• President Bush named Marine Corps General Peter Pace, who quietly helped to shape the Pentagon’s role in the war on terrorism, to be the next Chairman of the Joint Chiefs of Staff. General Pace, 59, would succeed Air Force General Richard Myers. He is expected to win easy Senate confirmation.

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