He indicated that Britain’s commitment in the region could last longer than previously expected, but admitted that some regiments returning home would have to return to Iraq after a few weeks of rest and recreation. Defence sources said the returning troops would be given six weeks off before returning to Iraq.

Mr Hoon also disclosed that he had been forced to issue compulsory call-up notices to another 1,200 reservists to reinforce the 3,000 already serving in Iraq. He said that further call-ups would be necessary as reservist units that were sent to Iraq for the war were brought home and demobilised.

The Defence Secretary acknowledged in a written Commons answer that the Armed Forces were facing “significant pressures” because of the continuing commitment in Iraq.

His admission followed the warning on Monday from Admiral Sir Michael Boyce, the Chief of the Defence Staff, that British troops could not take on another war this year or next.

In the week of his retirement, he said that the deployment of 45,000 military personnel to the Gulf had created serious overstretch. Mr Hoon has told the Commons in the past that he had every intention of bringing the troops home as soon as possible. But yesterday he gave warning that there would be an “appropriate” British military presence in Iraq “for as long as necessary”.

Mr Hoon said that 3,500 troops, mostly from 7th Armoured Brigade and 16 Air Assault Brigade, would now be withdrawn. The whole of 3 Commando Brigade Royal Marines, which had served the longest in the region, would also come home this month. This brought the total withdrawals announced yesterday to about 6,500 ground troops.

However, the same units from 7th Armoured Brigade — the Desert Rats — are to be affected by the “about-turn” deployment decision. The 2nd Royal Tank Regiment, the 1st Battalion The Light Infantry, 26 Regiment Royal Artillery and 38 Engineer Regiment will all have to go back after their six weeks off. They will leave their Challenger 2 tanks, Warrior armoured fighting vehicles and AS90 heavy guns behind in Iraq.

In the meantime, the 6,000-strong 19 Mechanised Brigade, based in Catterick in North Yorkshire, is to be sent out to Iraq to take over some of the Desert Rats’ functions — and their equipment. It would be too expensive to “desertise” all 19 Brigade’s tanks and Warriors to cope with the conditions in Iraq.

Mr Hoon also announced yesterday that HMS Ocean, the helicopter carrier, would be returning home, along with HMS Edinburgh, a Type 42 destroyer, and four Royal Fleet Auxiliary support ships.

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