Foreign Secretary Jack Straw has paid tribute to outgoing US Secretary of State Colin Powell and quelled fears about the impact of his departure.
Colin Powell submitted his resignation on Friday
Mr Straw praised Mr Powell's efforts to win consensus on the Afghanistan war and in ensuring the UN debated Iraq.
Liberal Democrat leader Charles Kennedy has said Mr Powell was a cautionary presence who needed to be replaced by a "sensitive internationalist".
But Mr Straw said Mr Powell had always acted with President Bush's approval.
US State Department officials say Mr Powell will resign as soon as a successor is found.
The 67-year-old, long rumoured to have planned to serve only one term, is said to have submitted his resignation to President George Bush last Friday.
Prime Minister Tony Blair saluted Mr Powell as "a remarkable man and has been a good
friend to this country over a very long period".
In a news conference on Monday afternoon, Mr Straw said his US counterpart would have a busy schedule before he left office.
"It has been a joy to work with Colin Powell and over the last three-and-a-half years I have come to know him better and better," he said.
Mr Straw said Mr Powell was a "unique individual" who had made the transition from being a great soldier to being a great diplomat.
He highlighted his "indefatigable efforts" to get resolution 1441, threatening Iraq with "serious" consequences if it did not meet its weapons obligations, through the UN Security Council.
He had also encouraged President Bush to be the first US leader to support a viable separate Palestinian state.
"Colin has great energy, he is a man with utmost integrity and he is huge fun," he continued.
Asked about concerns that the US administration had lost a moderating influence, Mr Straw said the UK would work closely with whoever emerged as Mr Powell's successor.
He argued that President Bush alone had decided to take the Iraq issue to the UN and to make his statements on the Middle East peace process.
"Everything that Colin Powell has done, he's done with the full authority of the president," he added.
'Force for good'
For the Lib Dems, Mr Kennedy said Mr Powell's exit was a disappointment coming at a an extremely difficult time in world affairs.
"In so many senses we are losing a friend at court," he said.
"He was widely perceived as the cautionary presence and voice at the
top table of the first Bush administration. History will be the judge
as to the extent, left to his own instincts, the Iraq war might have
Mr Kennedy said the choice of the new secretary of state would be crucial.
"We can but hope for a sensitive internationalist
and not someone who will be intent on pursuing still more of a
radical, neo-conservative and ultimately isolationist foreign policy," he said.
Conservative shadow foreign secretary Michael Ancram said simply of Mr Powell: "He has been a force for good in a turbulent world and he will be missed."