The U.S. president's hand-picked ambassador to the United Nations is resigning after he failed to win Senate approval, the White House said Monday.

John Bolton was U.S. President George W. Bush's own selection for the role, but the nomination has languished in the Senate foreign relations committee for more than a year, blocked by Democrats, and even some Republicans.

American ambassador to the United Nations, John Bolton, resigned Monday.
(David Karp/AP)

In a letter addressed to Bush on Friday, Bolton explained that he is only resigning after careful consideration.

"It has been a great honour and privilege for me to serve in your administration," the letter reads. "Your leadership has been critical in safeguarding America's values and interests in a time of peril and challenge."

Bush gave Bolton the job temporarily in August 2005, while Congress was in recess, which allowed him to take the job without Senate approval. The appointment expires when Congress formally adjourns, no later than early January, and Bolton will step down then.

"Ambassador Bolton's confirmation was blocked by a Democratic filibuster, and this is a clear example of the breakdown in the Senate confirmation process," said White House deputy press secretary Dana Perino.

"Nominees deserve the opportunity for a clean up or down vote. Ambassador Bolton was never given that opportunity."

The resignation is "a big defeat for the president," said CBC Washington correspondent Henry Champ.

"The president snuck the ambassadorship over to Bolton," Champ said. "It angered the Democrats and today is payback time."

Critics disliked Bolton's brusque style and questioned whether he could bring reform to the UN. But as recently as last month, Bush said he would not relent in his defence of the appointment.

'(Bush) looked at the cards and they simply were reading, "Bolton must go."'-CBC correspondent Henry Champ

The White House resubmitted Bolton's nomination but, with Democrats capturing control of both houses of Congress in last month's mid-term elections, the chances confirmation appeared slight.

The incoming chairman of the Senate foreign relations committee, Democratic Senator Joe Biden of Delaware, said he saw "no point in considering Mr. Bolton's nomination again."

Without the chance to make another recess appointment, the White House was believed to have been exploring other ways to keep him in the job, perhaps by giving him a title other than ambassador.

Champ said the White House realized over the past few days that keeping Bolton would be impossible.

"(Bush) looked at the cards and they simply were reading, 'Bolton must go,' " Champ said.

Bush accepted the resignation, and planned to meet with Bolton and his wife Monday in the Oval Office.

Perino said Bolton has had great accomplishments at the UN. He assembled coalitions that addressed North Korea's nuclear activity, Iran's uranium enrichment and Darfur's civil violence, she said.

She said Bolton made UN reform a top priority because the U.S. wants to see a more credible and effective UN.

"Ambassador Bolton served his country with distinction and he achieved a great deal at the United Nations," she said.

With files from the Associated Press