As many as 35 were categorised as either undeployable or with limited deployment potential. Two Royal Marine commando units have also reported hearing defects.

The MOD said that noise-induced hearing loss was a serious risk, but that earplugs and ear defenders were issued to all troops. However, unlike the US Army, ear protection is not enforced except in helicopters.

The Royal National Institute for Deaf People (RNID) has launched a campaign to improve ear protection for the Armed Forces. Angela King, a senior audiology specialist at the institute, said: "Repeated exposure to gunfire, even if it doesn't do traumatic mechanical damage, could lead to permanent hearing loss because of damage to the sensory cells in the inner ear.

Max Arthur, author of Forgotten Voices of The Great War, said: "No soldier wants to admit that he is going deaf – it would mean no longer being of use in battle. Without full hearing you put yourself at a disadvantage – and those around you. Being close to a severe blast can also bring the loss of silence that comes with tinnitus. No one has the perfect answer. Your ears may save your life; if they are overprotected that may cost you your life."