People with haemophilia A - the most common form of the disease - cannot produce factor VIII, which is responsible for forming blood clots.

This causes them to bleed for longer than normal. Although small cuts and grazes are not great problems, internal bleeding into joints, muscles and soft tissues can be life-threatening.

But scientists have successfully used the virus to raise factor VIII levels and maintain them for 11 months.

The research was conducted on mice and the researchers hope it will bring them closer to their goal of a one-dose cure for the disease.

Painful complications

People with severe haemophilia typically have less than one per cent of the normal amount of factor VIII in their blood.

They can suffer complications such as spontaneous bleeding, diseased joints and chronic arthritis.

The new gene therapy succeeded in raising this level to 20%, meaning that such extreme complications would become a thing of the past if successful in humans.

Dr Christopher Walsh, director of the University of North Carolina's gene therapy centre, led the research.

He said: "Although the [blood] levels of factor VIII are not at 100% using AAV gene transfer, they would be clinically beneficial to patients with haemophilia A.

"For most of the patients, if you can get their factor VIII levels to greater than five per cent constantly, they wouldn't have joint disease, the chronic arthritis, the deformities, and other problems haemophilia patients go through by the time they're in their twenties and thirties."

Genetic problem

In haemophilia patients, a faulty gene is thought to cause the low levels of factor VIII. The researchers increased levels by attaching a functioning version of the gene to a virus.

The virus then replaces the faulty gene with the good one. A single injection of the virus led to a rise in factor VIII levels that have now remained stable for 11 months.

"The goal is one dose - that essentially one dose delivered by gene therapy would be all that's needed to be physiologically cured of future bleeds," Dr Walsh said.

Haemophilia affects about 10,000 people in the UK. Most of them are men, although the genetic fault is carried by women.

Infection risk in current treatments

Current treatments involve expensive injections of factor VII or blood transfusions.

This poses a risk of infection - the Haemophilia Society says contaminated blood products have infected more than 1,200 people with the HIV virus and over 3,000 with hepatitis C.

Karin Pappenheim, chief executive of the society, said the research was promising but not conclusive.

"It's still far too early to say whether a treatment that has proved successful in mice will transfer into an effective treatment in humans," she said.

"However, should gene transfer eventually become a reality for the treatment of haemophilia in humans, it could make a significant and positive difference in quality of life and prevent the pain and disability the condition causes for many."

Dog tests next

The next step for the US scientists is to test the virus on dogs.

In January, another team of researchers at the centre successfully tested a cure for haemophilia B on dogs.

Haemophilia B is more severe than the A-type, but occurs only a tenth as often.