Geoff Hoon, the Defence Secretary, told MPs today that 850 British troops will redeploy from Basra to an area south of Baghdad.

Mr Hoon finally confirmed the decision to accede to a US request for an armoured battle group after receiving advice from army chiefs that there was a "compelling military operational justification" for it.

Soldiers from the Black Watch battalion will replace a US Marine unit earmarked for "other purposes" - widely understood to mean a US offensive on the rebel stronghold of Fallujah.

The Cabinet earlier unanimously supported the redeployment on the recommendation of Chief of Sir Michael Walker, the Chief of Defence Staff.

His approval came after receiving the report of a British reconnaissance team which had scouted the area, which is believed to be around the town of Iskandariya, some 20 miles southwest of Baghdad, in an area dubbed the "Triangle of Death"

General Walker said that the troops would remain under British command, but would be under the tactical control of the US Marines - a joint command arrangement that had been tested before in Iraq and elsewhere.

He also said that the agreement with the US Military was for a 30-day limit, although contingency planning was already under way in case that limit was exceeded.

"The Black Watch are a highly capable, well trained and hardened battle group and well suited to the tasks they have been asked to undertake," he said.

"In sum, it represents an important and a viable military task that we are well placed to carry out and that is why I recommended acceptance to the Secretary of State."

Mr Hoon gave no timetable for the redeployment, which would involve 850 troops including support units, but said it would last "weeks rather than months".

The Black Watch battlegroup will include three companies of armoured infantry, totalling some 500 men, equipped with 50 Warrior armoured troops carriers.

They will be supported by an armoured reconnaissance unit from the Queen’s Dragoon Guards, with about 100 men and 12 Scimitar armoured fighting vehicles, and around 50 light infantry from 40 Commando. In addition there will be back-up support from logisticians, communications experts and medics.

More than 40 Labour backbenchers, supported by the Liberal Democrats, have signed a motion calling for any redeployment to be put to a parliamentary vote because it would significantly widen Britain's role in the conflict.

But Mr Hoon said the decision had been an operational one and commanders on the ground had deemed that involved an "acceptable" level of risk.

The 1st Battalion The Black Watch had been expected to return to its base in Perth, Scotland, within weeks but will now serve out its full six-month term of duty before returning home by Christmas.

Mr Hoon denied a report in The Times this morning that the Government was planning to send an extra 1,300 troops to deal with an expected surge of military activity before nationwide elections scheduled for January, although he said it was "likely" some extra forces might be needed.

He added that the redeployed Black Watch troops would remain under UK rather than US command.

Mr Hoon said that routine changes in the UK military in Iraq meant that the 1st Battalion Scots Guards - equipped like The Black Watch with Warrior armoured vehicles - would take over as the reserve force for the UK in its operational command area around Basra.

Nicholas Soames, the Shadow Defence Secretary, said the Conservatives supported the redeployment as "a necessary operational military contribution to the coalition's efforts to bring peace and stability to Iraq ahead of the January elections".

Mr Soames sought assurance that the troops would have sufficiently robust rules of engagement to carry out their task - and that the judgment of commanding officers would not be undermined by the possibility of international legal action against them.

He also unsuccessfully appealed to the Defence Secretary for the "exceptionally shabby way" in which the decision to redeploy the troops had been handled - increasing the uncertainty for soldiers' families.

Britain currently has around 8,500 military personnel assigned to Operation Telic, the codename for the British mission in Iraq. Most are based around the southern city of Basra, which has also seen an upsurge in violence in recent weeks but is generally considered more secure than the US-controlled areas around Baghdad.

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