"After careful evaluation, the chiefs of staff have advised me that U.K. forces are able to undertake the proposed operation, that there is a compelling military operational justification for doing so, and that it entails a militarily acceptable level of risk for U.K. forces," Hoon said.
Some 850 troops will redeploy from around Al-Basrah in the south to an area near Baghdad.
Hoon said the troop deployment will last for a "limited time" only. Britain's chief of defense staff, General Michael Walker, said later it would be for 30 days, at most.
In Washington, the move was welcomed by State Department spokesman Richard Boucher.
"We welcome the announcement that Defense Minister Hoon made regarding redeployments," Boucher said. "And I think it demonstrates, once again, that Britain has been a staunch supporter of the multinational effort to help the Iraqi people establish control of their own country."
But at home, the decision by Prime Minister Tony Blair's government is deeply controversial.
The troops will be moving from the relatively peaceful south of Iraq to a more dangerous area, where they may come under hostile fire.
There are also concerns that the troops are redeploying because the United States is preparing a major assault on the insurgent stronghold of Al-Fallujah.
For weeks, U.S. airstrikes have targeted what the military says are militant hideouts in that city.
But Iraqi doctors say the airstrikes have already caused significant civilian casualties. In London, some fear the redeployment could mean that Iraqis will "unjustifiably associate" the British troops with civilian suffering.
But above all, critics are suspicious of the timing of the U.S. request. They say it comes remarkably close to the U.S. presidential election on 2 November. Could it, they ask, have been designed to give a boost to Blair's ally, U.S. President George W. Bush?
But Hoon said the move is a purely military matter and is aimed at stabilizing Iraq so it can hold elections in January.
The troops involved -- mainly from Scotland's Black Watch regiment -- are well equipped and experienced, having taken part in the overthrow of Saddam Hussein last year.
British military analyst Christopher Langton says the troops will prove useful, even though they number just a few hundred -- compared with more than 100,000 U.S. troops.
"The U.S. has about 130,00 troops, that is true. [But] many of the frontline units are committed to combat operations, and many of the non-frontline troops are reservists -- by that I mean National Guard units -- or they are in administration or logistics and therefore not particularly suited to frontline operations," Langton said. "Whereas the Black Watch have, for example, the same sort of skills that the U.S. forces have that they are relieving."
But he added that the chain of command is still unclear and must be clarified soon.
The troops are reportedly to remain under overall British command, although U.S. commanders will have a say in their day-to-day operations.