By Jonathan Marcus BBC defence correspondent

If war does come in the Gulf then Britain looks set to make the largest contribution of any of America's allies.

Indeed, few other countries have yet made any definite practical commitments.

But Britain's options are being constrained by both practical and diplomatic considerations.

This is not about numbers for numbers sake. British combat forces would be expected to fight hard.

The announcement by the defence secretary on Tuesday, which is expected to include the call-up of specialist reservists and details about the deployment of a Royal Navy task group to the Gulf, will be presented as precautionary measures.

These will give Prime Minister Tony Blair a range of military options should war come.

The Royal Navy's task group will be built around the carrier HMS Ark Royal - probably carrying helicopters and Royal Marines.

Additional Marines could be despatched on board the helicopter assault ship HMS Ocean.

Other British light forces - paratroopers for example - are also reportedly stepping up their training.

Fighting hard

All this suggests Britain may eventually opt for the despatch of a relatively "light" force to the Gulf rather than the armour-heavy, reinforced brigade that has been much spoken about.

Britain's practical contribution to any conflict falls into two broad areas.

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Those capabilities that actually assist the Americans qualitatively - for example elite special forces, air-to-air refuelling tankers, naval mine-hunters and so on.

These are all things that the Americans have probably asked for.

RAF tanker aircraft for example have the same refuelling system as used by the US Navy, greatly assisting the Pentagon, since the US Air Force deploys a different system.

But then there is the quantitative element - ground combat units and additional aircraft - that signals both the strength of Britain's commitment to act alongside the United States and its willingness to play a significant part in the ousting of Saddam Hussein.

This is not about numbers for numbers' sake.

British combat forces would be expected to fight hard.

Tony Blair does not want to spread his forces too thin

But does the Pentagon really need an under-strength British armoured division operating different equipment with all the logistical and support headaches that this brings?

Indeed, sending armoured forces presents some immediate problems for the British Government.

Potential entry points

Much of their equipment needs to be adapted for desert conditions.

And it might not be ready in time for any US-led ground assault.

British armour still may go to the Gulf.

But ministers may decide that the most visible initial British involvement will be this maritime component capable of assisting the US in any amphibious operation in southern Iraq.

This is one of three potential entry points into the country - the others being Iraq's land borders with Turkey and Kuwait.

British armour would be expected to deploy to Kuwait.

British troops also have some experience in operating in northern Iraq from their humanitarian operation in Kurdish areas.

But with money tight and public opinion unsteady, Tony Blair does not want to spread his forces too thin, eager to concentrate Britain's effort where relatively small numbers can provide the biggest military and diplomatic punch.