The US team says the virus is carried by 20 to 30% of the population, making them more susceptible to developing the killer disease.

Unusually however, the virus cannot be passed between humans, but instead is part of a person's genetic make-up and is passed from parent to child.

There is now the prospect of both accurate screening tests to find women at increased risk of developing breast cancer, and perhaps even a vaccine to counter its effects.

Professor Robert Garry told the 11th International Congress of Virology in Australia that the virus was a "significant find".

Larger trials needed

But he conceded that his research focused on only a very small number of women, and that larger studies would have to be completed to verify the results.

He said: "If it is true this virus has a role in human breast cancer, then this gives us a target to go after.

"Right now we diagnose the cancer after it has already developed...we'd like to be able to prevent breast cancer.

The virus is similar to one which causes cancer in mice

"It's virus, so presumably we can make a vaccine against it."

The virus is the product of a search that started back in the 1940s, when scientists discovered a virus that appeared to cause breast cancer in mice.

However, searching for its equivalent in humans was likened to finding a "needle in a haystack", as an estimated 50,000 similar viruses can live in the human body.

Technology aided search

Recently however, advances in technology allowed Professor Garry to take another look, and the suspect virus was found in a high proportion of breast cancers and benign tumours.

The study looked at 30 breast cancer samples and found that 85% of them contained the virus, as opposed to only one in five samples of healthy tissue.

"It is almost inconceivable to me that it will not be responsible for some percentage of human breast cancers," said Professor Garry.

UK experts gave the research a cautious welcome.

Professor Nick Lemoine, Director of Imperial Cancer Research Fund's Molecular Oncology Unit at the Hammersmith Hospital, said: "It is a fascinating observation and it will be interesting to see whether these results are reproduced in European populations."