Mr Bush, who received Mr Bolton’s resignation letter on Friday, said he was “deeply disappointed” that a “handful” of senators had blocked the appointment. But the resignation reflected the new reality in Washington of a greatly weakened president who, with Democrats now controlling the House and Senate, can no longer count on getting what he wants.

He is the second high-profile casualty of that change in the balance of power in last month’s mid-term elections following Mr Bush’s removal of Donald Rumsfeld as Secretary of Defence. Confirmation hearings for his successor Robert Gates begin tomorrow.

Mr Bolton, a blunt, irascible hawk who earlier in his career expressed contempt for the UN, was unable to get Senate confirmation last year and was installed to the post by Mr Bush in August 2005 by means of a recess appointment, when Congress was not in session. Recess appointments are temporary, and without formal confirmation this month Mr Bolton’s tenure at the UN will expire when the current session of Congress adjourns on January 3.

As recently as last week Mr Bush was adamant that Mr Bolton should remain at the UN despite the fact that he lacked the votes on the Senate Foreign Relations Committee to move the nomination to a full floor vote.

The White House had been looking for yet another way around the impasse, one option being to appoint him as an “acting” ambassador for another few months. But Mr Bolton’s fate was effectively sealed with the Democrats’ takeover of the Senate after last month’s mid-term elections. Joe Biden, the incoming chairman of the Foreign Relations Committee, said he saw “no point in considering Mr Bolton’s nomination again.

The final blow came after Lincoln Chaffee, a moderate Republican on the committee who lost his Rhode Island seat on November 7, said he would not vote for Mr Bolton if the nomination was considered in the final weeks of this congressional session.