By Jonathan Marcus BBC defence correspondent

British Defence Secretary Geoff Hoon described Monday's announcement as "no ordinary measure" and indeed he might.

This is a surprisingly large British deployment of troops.

Indeed, it is pretty well everything that Britain could reasonably send to the Gulf and support in the field given the military's other commitments - not least the fire brigade strike.

UK troops in the Gulf

Royal Navy Task Group has set sail Personnel include Royal Marines from 3 Commando Brigade Headquarters 1 (UK) Armoured Division Support from the Desert Rats 16 Air Assault Brigade 102 Logistics Brigade 120 Challenger tanks 150 Warrior armoured personnel carriers

Click here for map of UK military build-up

In proportional terms it is a larger deployment in relation to the overall size of the British Army than was the force sent to the last Gulf War in 1991.

Announced on Monday was a force of some 26,000 troops that must be added to the several thousand-strong amphibious force of Royal Marines already on their way to the region.

It comprises a much reinforced armoured brigade - the 7th Armoured Brigade known as "the Desert Rats" and elements of the headquarters of One UK Division.

Known in military jargon as "a square armoured brigade" it is made up of four regiments or battalions; two armoured and two mechanised infantry.

In terms of military punch that is some 120 Challenger Two tanks; 150 Warrior combat vehicles and 32 AS-90 self-propelled guns.

Additional artillery will go with the air-mobile brigade along with its three infantry battalions and their integral helicopters and anti-tank units.

The deployment list is completed by a significant part of a Logistics Brigade to support the British force in the difficult desert conditions.

The timing of this announcement is also significant.

Britain's Labour Government has faced an undercurrent of criticism from within its own ranks to Prime Minister Tony Blair's resolute backing of the American line.

Scepticism is high about involvement in any war

Large sections of British public opinion are also sceptical about the country's potential involvement in a major war.

By the time the chief UN weapons inspector Hans Blix reports back to the Security Council at the end of this month British troops will be well on the move.

They should be established on the ground - probably in Kuwait - by mid-February.

There is still time to avert a war.

Geoff Hoon told the House of Commons that there had been no commitment to any specific course of military action.

But time is running out. Many pundits believe that without some dramatic change of attitude on the part of Saddam Hussein, an air war could be under way by early March.

Britain's deployment signals Tony Blair's desire to ratchet up the pressure on Saddam Hussein.

But it also demonstrates that Britain intends to take a significant part in the fighting if and when war actually comes.