The findings could change the way people with head injuries are treated.

Previous research had suggested that people with head injuries cannot get post traumatic stress disorder (PTSD) because they are unable to remember the event concerned.

But research presented at the British Psychological Society's conference in Belfast suggests that only people who have very severe injuries, causing long-term memory loss of a week or more, do not suffer PTSD.

The disorder commonly includes flashbacks to the accident which led to the injuries, such as dreams, waves of feeling and involuntary images.

Mild injuries

Dr Graham Powell, a consultant clinical psychologist, studied 802 people with head injuries and compared them to 213 adults with PTSD.

He found that 39% of the people with mild head injuries - those who lose consciousness for up to an hour after an accident - were just as likely to suffer PTSD as other people who had experienced a traumatic event.

Twelve per cent of those with very severe head injuries also had PTSD symptoms.

He said the symptoms are broadly similar between those who suffer head injuries and those who do not.

Thousands suffer head injuries

However, Dr Powell said patients may have flashbacks that relate to before or after the accident, for example, waking up in an intensive care unit.

Some people may also become obsessed with retrieving gaps in their memories.

He said his findings had implications for the treatment of people with head injuries.

"People often do not ask about trauma because they assume head injury patients cannot recall their accident.

"Lots of people therefore have quite a difficult time psychologically."

Thousands of Britons suffer head injuries every year and more than 40,000 of them are children.

Invisible disability

The Children's Head Injury Trust says many are of a serious nature with consequences which will trouble the victims for the rest of their lives.

It adds that increasing numbers of people are surviving with severe head injuries which may or may not be visibly detectable.

"This means that their problems may be little understood by the community at large," it said