WOMEN shift workers and airline stewardesses may be at increased risk of breast cancer, a British scientist warned yesterday.

Research should be mounted into the possible link between cancer, jetlag, and the loss of the night-watch hormone melatonin, said Dr Anthony Mawson.

High-altitude radiation was widely assumed to be responsible when research in Finland indicated three years ago that long-haul stewardesses doubled their risk of breast cancer over 15 years.

Now Dr Mawson, a UK emigre and epidemiologist with the Carolinas Healthcare System in Charlotte, North Carolina, believes the biological upheaval of jetlag may be responsible.

While that probably would not be enough to affect women airline passengers to the same extent, a more general link with disruption to the circadian cycle - the body's internal clock - widens the implications to thousands of other women, like nurses, who work shifts.

In a research letter to the Lancet, Dr Mawson said the effect would lead to lower production of melatonin, which is produced by the pineal gland in the brain and mostly secreted between the hours of 9pm and 8am.

He said being awake during normal sleep time and trying to sleep during normal waking time would be expected to decrease melatonin production, which was known to be reduced by bright light during normal sleep time.

Experiments had shown that melatonin is able to suppress the majority of breast cancers by lessening the effect of the female sex hormone oestrogen.

Dr Mawson told The Herald yesterday: ''We could be looking at the different pieces of a jigsaw here, and I think it would be a good idea to carry out some research to see if they fit together.

''If the problem was restricted to flight attendants then the numbers involved would not be large in absolute terms. But if it applies to female shift workers generally, then the implications are far wider. I don't think very much is known about the relationship between breast cancer and shift working.''

He said his theory could be tested with a controlled trial involving a few hundred women with breast cancer.

About two-thirds of breast cancers are partly triggered by the action of oestrogen on breast tissue.

Dr Mawson wrote: ''These data suggest that work-associated disturbances in biological rhythms in general and melatonin production in particular are involved in breast cancer, and may contribute to the increase of about twofold in risk of breast cancer among female flight attendants.''

Professor Gordon McVie, director-general at the Cancer Research Campaign, said: ''I think this is an interesting hypothesis and it could very easily be put to the test.

''The radiation theory hasn't gone away, but what I find difficult to explain is why there isn't an excess of leukemia, because that is the prime cancer associated with radiation.''

Dr John Toy, director of clinical research at the Cancer Research Campaign, said scientists were just starting to discover the effects of melatonin on breast cancer.

He confirmed that laboratory experiments had shown melatonin could stop tumours spreading. It was possible that the hormone might become a new weapon against breast cancer in the future - possibly as a prophylactic.

At present melatonin is readily obtainable over the counter in America, but is not licensed in the UK.