While acknowledging the difficulties the troops face, George Robertson told BBC Radio 4's Today programme that Nato is meeting this week to consider how many more will be needed.

Original estimates of about 25,000 troops would have to be increased if there were not a "completely permissive environment", he said, adding that "lightly armed troops are not the answer to the problems of the Balkans".

The government has already promised to send 2,300 crack troops from the UK, including soldiers from the Royal Gurkhas and the Parachute Regiment.

UK soldiers are a key part of Nato's bombing campaign on Serb targets, and have helped build facilities for refugee camps. The campaign has been running for 62 days.

But the strain Mr Robertson spoke of is a "real, real problem", according to Paul Beaver, a spokesman for Jane's Defence Weekly. He told BBC News Online it is caused by lack of funding and recruits.

Blaming the government's money-saving Strategic Defence Review (SDR), he said it was the wrong time for the Ministry of Defence (MoD) to be saving cash when troops were so vitally needed.

The SDR is cutting defence spending by £2bn over the next two years.

"The air campaign in the Balkans alone costs the UK £3m a day, and it's still difficult to estimate the cost of the whole operation," he said.

Mr Beaver also said the SDR's disbanding of Territorial Army infantry battalions, since 1 April, was a bad move, making extra troops "unavailable".

A major complaint is that many soldiers are not seeing enough of their spouses and children. He said they are "rushed from one six-month assignment to the next, with only a fortnight off in between".

He said the officer in charge of the mine detection and disposal scheme in the Balkans had not had a break with her husband for several months, and was just one example of many.

"Although it does not affect performance, it does affect morale," he said.

"A lot of them are completely knackered."

The army currently employs 109,000 personnel, and knows its staff are under stress and "overstretched".

Apart from the US, the UK has sent more troops to the Balkans than any other country. Mr Beaver said this was because of their valuable experience of active service in Northern Ireland and Bosnia.

But it is not just morale that is low, he said.

After being in the Balkans for 17 days, soldiers receive 60% less overseas allowance. This has reportedly caused upset within the ranks.

German troops, many of whom are conscripts, are paid £40-a-day danger money. US troops have the interest on their savings doubled by the government while in active service, as well as also receiving danger money.

"But our troops, who are exceptionally well trained, lose out financially," said Mr Beaver. "It simply is not fair."

But an MoD spokesman was quick to hit back at the cash claims.

"Don't be misled," he told BBC News Online. "The reason they receive less cash in the Balkans is because the pound is worth less there.

"So, if they went to a country where the standard of living was higher than the UK's, their allowance would then go up."

He also said that all troops are paid the "X-Factor", which is an extra 12% on top of their salary in recognition of working in operational zones.

Major recruitment drives are also in place, he said, and 17,000 people were recruited into the army in the last financial year.

Extra troops drafted

To address the malcontent, troops are given a free 20-minute call home each week, free papers and postage. A number of sporting and social activities are also organised.

"Our troops are consummate professionals, and we know they will get on with the job in hand," he said.

"We are doing our best to keep their spirits up in the short term.

"In the longer term, we are taking on more troops than we have for 12 years, and we now offer six weeks' annual leave to everyone.

"But it may take a number of years to really solve the problem."