Derriford is one of six hospitals in the UK which employs MOD staff and many others are reservists who could be called up.

In the past week 33 non-urgent operations, such as hernia and varicose vein treatments, have been cancelled.

One patient who has had his operation put off said the experience had been "extremely stressful".

Overseas deployment

Glyn Warwick of Cawsand in Cornwall was told his non-urgent surgery could not go ahead, partly because the Stonehouse ward was closed.

He said: "I'm not the bravest of souls and this cancellation has caused a great deal of trouble."

The hospital's director of operations, Kim Hodgson, said there were a number of pressures on the hospital with the continuing presence of the Norwalk virus as well as the absence of MOD staff.

She said: "Obviously the departure of some of our military staff to overseas deployment has caused the closure of Stonehouse ward.

"But the Trust was aware of this exercise and has been for some time and we had made contingency arrangements."