WRVS is fighting to save its cafes and shops in hospitals because it can't afford to pay the same commercial rent as high-street chains.
The charity, whose volunteers staff outlets in 300 different hospitals across the country, has already been unsuccessful with three of its tenders this year. A decision on a fourth is still pending.
The fast-food chain Burger King already has branches at four hospitals in England, and WRVS fears the number of commercial outlets will increase as trusts are put under increasing financial pressure and forced to maximise profits.
A spokesman for the charity said: "The closure of our outlets will also endanger our other services, such as accompanying patients around the hospital.
"We will seek to demonstrate the added value that our volunteers provide, which in turn takes some of the pressure off staff."
One WRVS shop, at Great Western Hospital in Swindon, recently won an award from the local trust.
"We currently have 34 volunteers working there," said the spokesman.
"They contribute about 80 hours a week, which is the equivalent of two paid members of staff."