Reid said: "At a time when President Bush is asking our troops to shoulder a larger and unsustainable burden policing a civil war, his failed policies have left us increasingly isolated in Iraq and less secure here at home."
Edward Kennedy, a veteran Democratic senator, branded the British announcement a "stunning rejection" of Bush's strategy.
He said: "Now our country's strongest ally has rejected it.
"No matter how the White House tries to spin it, the British government has decided to split with president Bush and begin to move troops out of Iraq."
Joseph Biden, the Senate foreign relations committee chairman and Democratic presidential candidate, said that Bush should follow Blair's example and "start to draw down our troops from Iraq, not send more into the middle of a civil war".
"Has any of the Bush Iraq plans worked other than causing the worst destruction?"
Zaffar Zohair, Islamabad, Pakistan
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The US is currently undergoing a national debate on America's war in Iraq - driven by the new Democratic-controlled Congress - that is also increasingly defining the scope of the 2008 US presidential elections.
The House of Representatives last week managed to pass a non-binding resolution opposing Bush's surge of 21,500 extra troops into the country.
Bush administration representatives led by Dick Cheney, the US vice-president portrayed the move by Britain as a sign of that conditions were improving in Iraq.
"Now our country's strongest ally has rejected it"
Edward Kennedy, Democratic senator
Tony Snow, the White House spokesman, rejected suggestions the British announcement amounted to a timeline to withdraw forces from the country.
He said: "What you had is progress first and then the removal. This was not in response to any calendar that had been set by the parliament or by the prime minister."
Earlier, Cheney said the British move was a direct result of progress made in the British area of operations around the southern Iraqi city of Basra.
"Well, I look at it and see it is actually an affirmation that there are parts of Iraq where things are going pretty well," Cheney said.
War of words
While on his visit to Tokyo, the vice president told ABC News: "I think in fact if we were to do what Speaker Pelosi and Congressman Murtha are suggesting, all we'll do is validate the al-Qaeda strategy, the al-Qaeda strategy is to break the will of the American people."
Nancy Pelosi, the House Speaker and a California Democrat, fired back that Cheney's comments were "beneath" his office and said the US president should repudiate the remarks.
Pelosi said in a statement: "Vice President Cheney continues to question the patriotism of those of us in Congress who challenge the Bush administration's misguided policies in Iraq, but his latest attack is beneath the office of the vice president, especially at a time of war."
She said Cheney's comments would not dissuade Congress from pursuing a strategy to bring US troops home soon.
House representative John Murtha, a Pennsylvania Democrat and leading opponent of the war in Iraq, plans to advance legislation next month attaching conditions to new combat funds that would effectively block a troop buildup.