Accident victims and soldiers with severe head injuries could soon be treated with a contraceptive pill if the results of a major trial prove successful.

Research has suggested that progesterone, a steroid used to fool a woman's body into thinking it is pregnant, could also be used to protect damaged brain tissue.

An earlier study of 100 patients showed that injections of the hormone soon after a head injury safely reduced the risk of death and long-term disability.

The new Phase III trial conducted at 17 medical centres around the US will enrol around 1,140 brain damaged patients over three to six years.

The ProTECT III study will be led by scientists from Emory University in Atlanta, Georgia, where the neuro-protective properties of progesterone were first discovered more than 25 years ago.

Trial leader Dr David Wright, who announced the research at the annual meeting of the American Association for the Advancement of Science in San Diego, California, said: "No new treatment for severe TBI (traumatic brain injury) has been approved in over 30 years.

"With such promising success in laboratory testing and in our previous clinical trial, we hope to conclude in this national trial that progesterone - along with standard medical trauma care - works better than standard medical care alone in reducing brain damage caused from a TBI."

Traumatic brain injuries are common among road accident victims, and are also the "signature wound" of soldiers injured in modern conflict zones such as Iraq and Afghanistan.

Around two million adults and children in the US suffer a significant TBI each year, the researchers said.

The injuries led to 50,000 deaths and 80,000 new cases of long-term disability.