John Bolton has a history of combative criticism of the UN

Mr Bush said Democrats had forced him to bypass Congress by using "shameful delaying tactics" to prevent a vote.

"This post is too important to leave vacant any longer, especially during a war and a vital debate about UN reform," Mr Bush said.

After being sworn in in Washington, Mr Bolton went to New York to start work.

At times a stern critic of the UN, he will serve there until 2007.

Mr Bolton's candidacy did have the support of many senators but the nomination stalled because he would have needed 60 senators to force a decisive vote in the Senate.

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John Bolton

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In an announcement at the White House, President Bush praised Mr Bolton's experience in foreign affairs and blamed political opponents for the irregular nature of the appointment.

"The US Senate held thorough confirmation hearings, and a majority of US senators believe he is the right man for the job," Mr Bush said.

"Yet, because of partisan delaying tactics by a handful of senators, John was unfairly denied the up-or-down vote that he deserves."

Democrat discontent

At the president's side, Mr Bolton said he was "profoundly honoured" by his appointment.

He said he aimed to help reform the UN into a "stronger, more effective organisation, true to the ideals of its founders".

This is not the way to fill our most important diplomatic jobs

Democratic Senator John Kerry

They expressed concern he lacked the personal temperament for the UN job, following his combative criticism of the body. He is also considered to be a leading hawk on Iran, Iraq and North Korea.

Democratic Senator John Kerry, who fought for the presidency last year, acknowledged Mr Bush's constitutional right to make the appointment but condemned the move as the "wrong decision".

"It only diminishes John Bolton's validity and leverage to secure America's goals at the UN," he said in a statement.

"This is not the way to fill our most important diplomatic jobs."

'Well qualified'

However, Republican Senator John Cornyn defended the president for using his authority to "to end the obstruction against John Bolton", whom he described as "exceptionally well qualified".

The Bush administration was keen to have a UN envoy in place in time for the opening of this year's session of the General Assembly in September.

The US has backed reform of the UN Security Council, although it insists that plans to add 10 new members, including six permanent members, are flawed.

Mr Bolton served as under-secretary of state during Mr Bush's first term, with key responsibility for arms control and proliferation issues.