Hundreds of British soldiers will be redeployed in central Iraq in response to a US request, the defence secretary has confirmed.
The Black Watch is on its second tour in Iraq
Geoff Hoon told the House of Commons the deployment of about 850 Black Watch troops and support staff would last "weeks rather than months".
He said they would remain under the day-to-day command of British generals.
There were shouts of protest from the benches as Mr Hoon was questioned by opposition members after his statement.
Mr Hoon said the decision was based on military advice.
"After careful evaluation, the chiefs of staff have advised me that UK 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 UK forces," Mr Hoon said.
The 650-strong 1st Battalion The Black Watch and about 200 support personnel would move from their base in Basra to near the capital Baghdad.
Mr Hoon denied newspaper reports claiming Britain planned to send a further 1,300 soldiers to Iraq.
Falluja in sights
Redeploying UK soldiers to an area south-west of the Iraqi capital will free US troops for an anticipated assault on insurgent-held Falluja.
"We share with the Iraqi interim government and with our coalition partners a common goal of creating a
secure and stable Iraq, where men, women and children in towns like Falluja can
feel safe from foreign terrorists, from the kidnappers who murdered Ken Bigley and from other criminals," Mr Hoon said.
"Crucially, Iraqi Prime Minister Allawi and the interim government want to establish sufficient security for elections to take place in January."
BRITISH FORCES IN IRAQ
9,200 troops deployed to the Gulf, almost 7,500 in Iraq
1,400 of those are reservists
Most troops in Basra and al Muthanna provinces
1 Mechanised Brigade is currently 'lead formation'
6,315 troops from 10 nations also serve in the area
There were frequent shouts of protest during the exchanges that followed Mr Hoon's statement.
Shadow Defence Secretary Nicholas Soames said the Tories supported the redeployment as "a necessary military contribution" to the task of ensuring peace in Iraq in the run-up to January's elections.
But he said MPs would be relieved Mr Hoon had ended "the unnecessary and unacceptable confusion of
the last few days".
The Liberal Democrats' Paul Keetch said his party had not supported the war but accepted "we have responsibility for the people of Iraq".
However, he said, British troops should remain in the British sector.
"We do not today support this deployment."
Later the party's deputy leader Sir Menzies Campbell told BBC News 24 the deployment signalled support for the planned American assault on Falluja, in which many civilians were likely to be killed.
He said there was "no public support" for the move.
Sir Michael said there was a 30-day limit attached to the deployment.
"If time limit is breached, there will need to be a unit to replace the Black Watch," the general said.
The Prime Minister, Tony Blair, has pledged the troops will be home by Christmas.
A British reconnaissance team visited the deployment area and handed its assessment to military chiefs.
Sir Michael then made a recommendation to ministers on whether to agree to the move.
Opposition to the proposals saw 63 MPs sign a motion calling for a Commons vote on any possible movement of British troops. Mr Blair resisted such a move.