More than two-thirds of British troops returning from Afghanistan are suffering severe and permanent hearing damage, according to the most comprehensive study into one of the less well-known side-effects of the conflict in Helmand.

Internal defence documents reveal that of 1,250 Royal Marine commandos who served in Afghanistan, 69% suffered hearing damage due to the intense noise of combat. The findings indicate that complaints such as tinnitus or almost complete deafness among combat troops are considerably greater than previously reported. One audiologist said the report revealed that hearing loss was endemic among Afghan veterans, with many suffering defects that could bar them from frontline service.

The intensity of the conflict in Helmand and its close-combat fighting, roadside devices and the noise of low-flying coalition aircraft caused the problems, according to the Ministry of Defence study. The report, dated 7 December and written by military consultant surgeon Chris Pearson, warns that the known scale of the problem might prove to be the "tip of the iceberg" because only the most severe forms of hearing loss, grave enough to bar troops from frontline service, are officially reported.

Professor Mark Haggard, honorary vice-president of Deafness Research UK, which is working with the MoD to resolve the problem, said: "The issue has become systemic, endemic. Combat gunfire and explosions mean significant numbers are turning up with significant hearing problems."

Hearing tests were conducted on 1,254 troops from 42 Commando Royal Marines following their six-month tour in Helmand between April and October 2008. Analysis by the defence audiology service found that 865 of the Marines displayed signs of severe hearing damage caused by loud noise. Of these, 410 were classified as having more extreme cases.

Pearson, who is calling for further studies into the issue, concluded that 69% of the Royal Marines had "audiometric evidence consistent with NIHL [noise-induced hearing loss]".

British employers must not subject staff to noise levels over 85 decibels for prolonged periods. Haggard said the blast of a gun or "medium" explosion measured 140dB, equivalent to hearing a jet plane taking off about 40m away. The sound of a pneumatic drill at two metres distance measures 126dB while an alarm clock is 90dB. The normal pain threshold is around 120dB.

The MoD study, The Extent of Operational NIHL, also points towards major problems among reservists deployed to Helmand. Pearson's report reveals that one in 10 Territorial Army recruits also experienced a significant degree of hearing loss following their tour of duty which was "unlikely to be due to anything other than noise or blast".

Tests comparing the Royal Marines from Afghanistan with service personnel from an RAF base in the UK found that 17.7% had severe ear damage compared with 5.2% of the RAF personnel.

The MoD report also scrutinised in detail the medical records of 73 soldiers who had served in Helmand province and found that 67 had problems with their hearing. Of these 39 were diagnosed with noise-induced hearing loss with 16 new cases of hearing problems thought to be from "blast injury".

An MoD spokesman said they were trying out new ear protection for personnel on operations to protect them from the effects of loud explosions and gunfire. He said: "The system uses a custom moulded earplug with an inbuilt microphone to cut the noise impact of loud explosions while still giving the wearer the ability to hear colleagues. Feedback from trials with soldiers in training and in theatre have allowed the rapid development of new, easy-to-fit earplugs that stay in place."