Researchers say women who use the pill have a 25% reduction in hip fracture risk compared with those who had never taken it.

Those who have taken a high-oestrogen pill - which is no longer available because of health risks associated with it - have a 44% reduced risk of hip fracture.

However, bone experts warn that the results are not conclusive enough to warrant changes in practice.

No significant reduction

The study, published in The Lancet, was led by Dr Karl Michaelsson of the Department of Orthopaedics in Uppsala, Sweden.

Researchers collected data on all hip fractures occurring between October 1993 and February 1995 in six areas of Sweden.

Of the 1,327 cases studied, 11.6% had used oral contraceptives.

These were compared with a control sample of 3,312 women - 19% of whom had taken the pill.

The researchers found that women who stopped taking the pill before they were 30 had no significant reduction in suffering hip fracture.

Those who stopped between the ages of 30 and 40 had a small reduction, but the effect was most pronounced for women over 40.

The researchers suggest that this could be because the oestrogen in the pill protects the bones against degeneration in the years before the menopause.

Women are thought to reach peak bone mass at around the age of 35.

After that, they are estimated to lose between 0.5 and 1% of bone mass a year before the menopause.

Oestrogen supplements like hormone replacement therapy (HRT) are known to prevent the decrease of bone density which accompanies the menopause.

Oestrogen used in the pill is synthetically produced, unlike the natural oestrogen in HRT.

Timing

The researchers conclude: "These data demonstrate the importance of timing of clinical interventions in hormone-related processes such as osteoporosis.

Only a minority of women regain former mobility after a hip fracture

"Oral contraceptive use can offer a reduction in the risk of hip fracture in women, especially if used late in reproductive life."

However, Professor David Purdie, head of the centre of metabolic bone disease at Hull University and a council member of the National Osteoporosis Society, was sceptical about the research.

"The research is interesting, but the results are very weak. The researchers say there are lots of variables, for example, the women who took the pill could have had a higher bone mass to begin with."

"The evidence is not sufficiently strong enough for us to yet consider using the pill as a means of preventing hip fracture."

He called for a larger study to test the researchers' results.

According to the National Osteoporosis Society, one in six women suffer a hip fracture during their lifetime and the number of fractures is rising by 6% a year.

The problem costs the NHS �840m a year.