Blows to the head suffered when young may be putting almost one in 10 people in the UK at increased risk of a blighted or shortened life, research suggests.

A large-scale study has shown that even apparently mild head injuries can have potentially devastating long term effects, including an increased risk of disability, psychiatric problems, low educational attainment and even early death.

Many of those affected will have sustained traumatic brain injury (TBI) while playing contact sports such as football or rugby.

But scientists stressed the low "absolute" risk of being hit on the head and experiencing such repercussions, and said a balance had to be struck between the benefits and hazards of sport.

Researchers used national registries to study the progress of more than a million Swedes born between 1973 and 1985 up to the age of 41.

Of the total, 9.1% were found to have suffered at least one TBI caused by a blow to the head before their 25th birthday.

Compared with people who had not experienced a head injury, these individuals were 76% more likely to have qualified for disability payments, meaning they could not work.

They were also 58% more likely to have achieved a low level of education, almost twice as likely to have been taken to hospital for a psychiatric problem, and 72% more likely to have died prematurely before the age of 41.

Absolute risk was much lower, the study published in the journal Public Library Of Science Medicine showed.

Of the whole group, 10% of people who suffered a TBI underwent psychiatric treatment in hospital, 13.9% attained a low level of education, and 6.3% became disabled. The absolute risk of someone dying early after a TBI was only 1.6%.

Multiple brain injuries significantly increased the risks, as did experiencing a TBI at an older age.

The types of injury ranged from mild knocks that left a person temporarily dazed without losing consciousness, to serious brain damage leading to a lengthy stay in hospital.

To be included in the study, every patient had to have undergone an "alteration of consciousness" and received a hospital assessment.

Lead researcher Dr Amir Sariasian, from Oxford University, pointed out that many cases of mild brain injury went unreported.

He said: "This means we have essentially under-estimated our effects.

"The differences between the unexposed and exposed groups is likely larger than we have found here.

"To summarise, we found that even a single mild traumatic brain injury will predict poor adult functioning. The risk will increase with severity and recurrence and older age at first injury."

Head injuries are known to have a "primary phase" caused by the initial physical damage to nerve cells, and a longer lasting "secondary phase" linked to inflammation and toxins released into the brain. Scientists are still trying to understand the on-going effects, which may continue to kill off neurons for years.

The research could have implications for designing school playgrounds and safety equipment such as helmets, and perhaps even the rules of certain contact sports, it is claimed.

However Dr Peter Jenkins, who runs a head injury clinic at St George's hospital, London, and was not one of the study authors, urged parents not to be too concerned about the risks or to stop their children playing sport.

He said: "I think with everything in life there are risks and benefits. If you look at the absolute risks, they're not that large.

"It's a balance decision. We need to look at both sides.

"We don't know enough about sports-related head injuries. We need more research."

The prevalence of TBI recorded in Sweden was thought to reflect a similar incident rate in other developed countries, including the UK.

Comparisons made between siblings, who share 50% of their genes, indicated that genetics made some contribution to the trends seen in the study.

Professor Huw Williams, a neuropsychologist at the University of Exeter, described the study as "incredibly strong".

He added: "We don't know how the risk of TBI compares to other risks that children face. Issues such as adversity, poverty, abuse and neglect are all huge when it comes to brain development and all increase the likelihood of injury.

"This does not mean we should be banning children from playing contact sports but care is needed, especially in managing a child's return to play, so that they don't get injured again when they are still suffering from an injury (as they are very vulnerable then), and managing their return to school.

"We also need to make sure the rules are followed - more than 20% of concussion injuries in rugby appear to happen during foul play."