The Pine Street Clinic stands on possibly the prettiest street in the nicest town in America. San Anselmo is just north of San Francisco in wealthy but “alternative” Marin county (where the Prince of Wales will be this weekend). People here are 1960s and 1970s counter- culture types grown successful.

No surprise, then, that Pine Street is a “complementary” medicine centre. But work at the clinic is set to perplex and intrigue mainstream doctors the world over. A research paper — peer-reviewed by mainstream scientists and copper-bottomed by statisticians at the University of California, Berkeley — is to be published next year in a respected American medical journal. It suggests that the humble dog could bring about a revolution in the early diagnosis of cancer as well as embarrass the medical technology industry and the technology minded majority of the medical profession.

The claim of the Pine Street Foundation, which runs the clinic, is this: that dogs, given as little as three weeks’ training, can, by smelling samples of people’s breath captured in a special tube, detect cancers of the lung and breast even in their earliest stages — and can do so to a level of accuracy as good as and beyond that being achieved in conventional hospitals by the latest Cat, Pet and MRI scanners.

In a scientifically supervised trial involving 55 lung cancer patients, 31 breast cancer sufferers and a control sample of 83 healthy patients, five Pine Street dogs achieved an accuracy rate in detecting cancers of between 88% and 97%. The dogs even seem able to detect with near-certainty the first, symptom-free stage of lung cancer — a condition that technology can distinguish only to a lower level of accuracy.

There are many caveats in making crude comparisons but according to Cancer Research UK, Britain’s biggest cancer investigator, scanners typically costing £1.2m plus £300,000 a year to run reckon to detect early-stage cancers to an accuracy of 85% to 90% — a lower rate than Pine Street’s dogs.

The dogs are taught to indicate a “positive” find by holding out a paw for a treat. “Obviously, during double-blind testing the dogs received no treats as neither experimenter nor dog handler knew which breath samples were from cancer patients and which were from controls,” said Michael McCulloch, the clinic’s research director.

The dogs have also identified — without having been asked — early-stage lung cancer in a visiting Japanese television director and a malignant melanoma in a dog trainer. “Right now we’re talking with ethicists about what to do if a dog picks up a stage one cancer which doesn’t yet show up on a scan. People sitting in a restaurant don’t necessarily want to be told this,” said Michael Broffman, Pine Street’s founder.

The research findings will be published in March in Integrative Cancer Therapies, the journal of the University of Illinois, following a statistical analysis by the school of public health at Berkeley.

It represents a breakthrough for a small but persistent group of medics and other scientists in Britain and America who have been trying to demonstrate for nearly 20 years that dogs have exceptional diagnostic abilities with cancers ranging from malignant melanoma to breast cancer.

Last year a British project, the Amersham bladder cancer study, published a report in the British Medical Journal from a team at Amersham hospital, Buckinghamshire, that had trained dogs to detect cancer from urine to an accuracy rate of 41%.

This figure, low though it sounds, suggested that the dogs in the trial were correctly identifying urine samples at a three times greater-than-chance rate; one Amersham dog achieved 60% accuracy. The Amersham experiment gained much publicity, but was still regarded in medicine as more of a novelty than as a development of any practical use.

Another British team, which was working under Dr Barbara Sommerville at Cambridge University’s renowned veterinary school, ran out of its £42,000 grant money with its work incomplete and has just been denied a further three years’ funding.

There is also a rich vein of anecdotal evidence of untrained dogs discovering cancers. Gill Lacey, 46, is a Buckinghamshire mother and the editor of a charity magazine. She appears in a documentary film about Pine Street’s work (Can Dogs Smell Cancer? to be shown on BBC4 tonight) and believes her life was saved when she was 19 by Trudii, her parents’ dalmatian.