Hospitals in Afghanistan and the UK are under pressure from the large number of troops injured in Helmand province, a National Audit Office report has said.
The spending watchdog said the Helmand field hospital was close to capacity.
It praised "highly effective" treatment but said more military patients at the Selly Oak Hospital in Birmingham could mean civilians being moved elsewhere.
The warning comes as 4,000 UK troops are set to take part in an offensive in Helmand, southern Afghanistan.
British, US and Afghan forces are making final preparations for the assault on the Taliban.
The NAO said in general the government needed to plan more carefully for a rise in casualties in Afghanistan.
It raised concerns about contingency planning should Afghanistan casualty numbers overwhelm the specialist military facility at Selly Oak Hospital.
There is a voluntary agreement in the West Midlands to maximise the number of injured troops cared for at Selly Oak by diverting civilian trauma patients to other hospitals in the region.
If there is no room for military casualties at Selly Oak, they will be treated first in hospitals in the region, then nationally.
The NAO report said: "There is scope for improvement, for example by modelling the capacity required under different casualty scenarios and defining clear indicators for when each level of contingency would be required."
'First class treatment'
The NAO also said defence chiefs must do more to tackle a rise in the number of British troops in Afghanistan suffering from illness and minor injuries.
Edward Leigh MP, chairman of the Commons public accounts committee, said: "Given that the Selly Oak military hospital now has unrivalled expertise, we will be seeking reassurance that the quality of care for service personnel will remain high, even if Selly Oak becomes full.
"Disease and minor injury are currently having only a small effect on the capability of our forces but current figures show a rising trend in rates, a near doubling in three years.
"The MoD needs to find out why and improve prevention measures."
Veterans Minister Kevan Jones praised the "first class" treatment given to injured servicemen and women.
He said: "We proved we have the capacity to cope with an increase in casualty numbers in our response to the Panther's Claw operation last summer and we have contingency plans in place to deal with the unexpected."
The NAO estimated that the cost of medical care as a result of Britain's military operations was £71m in 2008-09.
It found that rates of minor injury and illness among troops deployed to Afghanistan nearly doubled between 2006 and 2009, increasing from 4% to 7% of the total.
The rate went up from 5% to 9% in Iraq over roughly the same period.
The watchdog estimated that 6,700 days had been lost in Afghanistan between October 2006 and September 2009 because of the increase.
It suggested that possible reasons for the rise could include the intensity of operations, basic living conditions at some forward-operating bases or improved reporting of medical data.
But the NAO also said the MoD's limited data meant it could not judge the significance of these individual factors.
A total of 522 UK service personnel were seriously injured on operations in Iraq and Afghanistan between October 2001 and October 2009, with the peak in July last year.
British troops have attended medical facilities about 125,000 times for treatment of minor injuries and illnesses since 2006, and some 6,900 have been evacuated back to the UK since 2003.
Meanwhile, changes to the financial compensation given to soldiers wounded in Iraq and Afghanistan are due to be announced following an independent review of the Armed Forces compensation scheme, which was introduced in 2005.
The MoD set up the review in the wake of growing public pressure to increase the payouts given to soldiers injured in battle.
These latest changes are believed to make the scheme both more generous and transparent.
Under the old rules, a soldier was compensated only for the worst three injuries from any one incident. The new rules will take into account all injuries suffered, with compensation on a sliding scale.
There will also be more generous awards for those who suffer from post-traumatic stress and a rise in the annual amount given to soldiers for their loss of earnings.
The BBC understands that the improved compensation scheme is likely to cost the MoD tens of millions of pounds.