Taser stun guns issued to police forces across the country have been declared broadly safe, following the largest independent study of injuries inflicted by the weapons.

A medical review of nearly 1,000 cases in which the controversial weapons were fired found that 99.7% of those hit were either unharmed or suffered only mild injuries, such as scrapes and bruises.

The majority of injuries were small skin punctures caused by metal darts fired from the guns.

But the study suggests there is a strong link between those more seriously injured and the number of times they are shocked by a Taser. The weapons incapacitate their targets with a 50,000-volt electric shock.

The guns are set to deliver a five-second shock before resetting, but in some cases officers have discharged them repeatedly to subdue people.

Three subjects in the study required hospital treatment after Tasers were used on them. Two suffered head injuries after falling down and a third was admitted to hospital with a rare condition called rhabdomyolysis, in which muscle tissue begins to break down. It is unclear if the condition was brought on by the Taser. Two other subjects died after being hit by a Taser, though autopsy reports indicated their deaths were not caused by the weapons.

More than 3,000 Tasers have been sent out to the British police force since 2004. Initially they were authorised only for use by trained firearms officers facing armed attackers, but those powers have since been extended to include incidents of severe violence or threats. A year-long trial announced last month will see Tasers issued to officers without firearms training in 10 UK regions, including Merseyside, West Yorkshire and London.

The move is being strongly opposed by the human rights organisation Amnesty International.

In the study, funded by the US National Institute of Justice, medical staff examined police and physician records every time a Taser was used by police in six regions across America.

William Bozeman, an emergency medical specialist who led the study at Wake Forest University School of Medicine in North Carolina, is due to present results of the investigation at a meeting of the American College of Emergency Physician's in Seattle today.

"This is the largest independent study to date, and the first to detail the medical effects of Tasers under real-world conditions," Dr Bozeman said. "This study promises to give us the best information yet on medical risks of these weapons."

Dr Bozeman said results from previous studies were limited by the use of animals and healthy police volunteers in training settings, not criminal suspects in real-world conditions.

"The Taser is a weapon and it can clearly cause injuries and even deaths in some cases," Dr Bozeman said. "The question is how likely is it to cause a significant injury, and whether that risk of injury outweighs the benefits it brings."

A spokesman for Amnesty International said the organisation has serious concerns over the safety of Tasers and has asked the government to reconsider its wider deployment of the weapons.