Detailed plans have been drawn up but ministers have yet to decide which to select. The options range from a limited contribution involving special forces and other specialist capabilities such as Tomahawk cruise missile-armed submarines and RAF air-to-air refuelling tankers, to a full-scale land force.

The Government is expected to go for a substantial ground force of up to 15,000 troops, as well as a carrier battle group, a Royal Marine commando, deployed on HMS Ocean, the helicopter carrier, and RAF Tornado GR4 bombers. While numbers have not been fixed, the total package could amount to between 20,000 and 30,000 personnel.

Intensive training is already going on in Germany, where the main elements of the proposed ground force are based. The units are all from 7th Armoured Brigade, known as the Desert Rats, and 4th Armoured Brigade, part of 1st (UK) Armoured Division.

The brigades are equipped with Challenger 2 tanks, AS90 self-propelled guns, multiple-launch rocket systems and Warrior armoured fighting vehicles. The ground force envisaged is described as an “enhanced armoured brigade” — one full brigade, the 7th, reinforced with elements from another, the 4th.

Regiments now preparing for war include the 1st Battalion The Black Watch, The Royal Scots Dragoon Guards, 1st Battalion The Royal Regiment of Fusiliers, 1st Battalion Irish Guards, 3rd Regiment Royal Horse Artillery, 26 Regiment Royal Artillery and 12 Regiment Royal Artillery.

Elements of 5 Regiment Royal Artillery and 32 Regiment Royal Artillery are also engaged in warfighting training in Germany, and a substantial number of signals, logistics and transport units have also been earmarked for Iraq.

Geoff Hoon, the Defence Secretary, had promised a significant contribution when he went to see Donald Rumsfeld, his American counterpart, in Washington about a month ago.

Now, for the first time, the Treasury will have to approve spending on spares, new equipment, and “desertising” Challenger 2 tanks, whose engines need to be protected from the sand in Iraq. The Royal Navy’s “R2” (high-readiness) aircraft carrier, HMS Ark Royal, is currently in Portsmouth undergoing maintenance after her involvement in the recent Nato exercise in the Mediterranean.

An Ark Royal battle group, with three or four warship escorts and one or two nuclear-powered submarines, armed with Tomahawks, is likely to be included in Britain’s offer. HMS Ocean is also in maintenance but will be ready for operations early in the New Year.

Under current plans, not yet approved by ministers, about 10,000 reservists may have to be mobilised. The mobilisation order which will have to be announced in Parliament, will be the clearest signal that Britain is preparing for war.

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