Britain is rumoured to have promised to send up to 50,000 troops as part of a 150,000-strong land invasion force for Kosovo.

NATO commander General Wesley Clark was reported to have discussed the possibility of a full-scale ground invasion with defence ministers from Britain, the United States, Germany, France and Italy. The New York Times said Clark had told them at an unpublicised meeting last Thursday that a force of up to 150,000 would be needed. The reports come within a week of British Defence Secretary George Robertson announcing that Britain would send an extra 12,000 men to Kosovo, taking its forces in the Balkans to over 19,000 out of a force of 48,000.

The promise of further troops was said to have been made at a private meeting on the fringes of a regular gathering of European defence ministers attended by US Defence Secretary William Cohen. The Pentagon had attempted to keep the Cohen trip a secret, but was forced to confirm that it had taken place. US defence officials insisted that the sole purpose of the meeting was to discuss how to raise and deploy the so-called peacekeeping force NATO had formally requested be assembled last week.

Spokesmen for Robertson denied that he had indicated Britain would contribute as many as 50,000 troops, but other defence ministry sources said Britain was continuing to plan for the possibility of sending additional forces.

The larger presence would be made up of 25,000 soldiers, 5,000 marines and up to 20,000 Navy and Royal Air Force personnel and logistics support staff. This would consign a quarter of all Britain's soldiers to the operation—the heaviest commitment for the British military since 1945. Though the numbers involved equal that of the Gulf War, Britain's armed forces were much bigger at that time.

In addition to the troop mobilisation, the army is preparing to call up civilian doctors and nurses to serve in Kosovo. Robertson wants call-up papers issued to hundreds of National Health Service doctors, nurses and other medical staff with military ties. Ministry of Defence and Department of Health officials are in discussions over the number of staff to be drafted from an already seriously strained National Health Service.

The Sunday Times quoted a senior government source saying: "Frank Dobson [Health Secretary] doesn't like it but he will have to lump it. By the time we enter Kosovo, we will find no local medical facilities and hordes of local people on the point of dying. Dobson will be told this is an international humanitarian emergency and he cannot argue with that."

The government would have to ask the Queen to sign an order activating provisions of the Reserve Forces Act 1996. Under this Act, members of the volunteer and part-time Territorial Army, and those who have served in the military within the past 10 years are compelled to leave their jobs and sign on full-time.

Preparations for the Gulf War in 1990 saw 390 medics called up. They complemented 700 Territorial Army and reservist volunteers with medical qualifications. In additional to the call-up of medical personnel, Robertson wants the former soldiers and Territorial Army infantrymen to be drafted into Bosnia so as to release regular troops there to go to Kosovo.

Officially, the Ministry of Defence maintains that "there are no current plans to call up reservists en masse". But sources in Whitehall say a decline in army numbers and a crisis in recruiting army medics made the call-up essential to preparations for the entry of an international security force into Kosovo. The army is currently running at about 20 percent of its full complement of doctors, with only 500 qualified personnel.

International Development Secretary Clare Short gave an indication of the timetable being pursued by the British government. Claiming that, "The only real way to bring humanitarian relief to those inside [Kosovo] is to succeed militarily," she said planning for the return of the refugees was "intensifying... knowing that winter will begin in September".