Epsom salts injected into the bloodstream can more than halve the risk of a common pregnancy complication becoming life-threatening, doctors revealed today.

Researchers said the findings from a major international trial involving more than 10,000 women were so dramatic they were likely to have an instant impact on the treatment of pre-eclampsia.

The three-year £2.5m "Magpie" study was conducted in 33 countries, including the UK. It showed that women given magnesium sulphate - commonly used as Epsom salts to soak away aches and pains - had a 58% reduced risk of pre-eclampsia progressing to eclampsia.

Pre-eclampsia, a condition that affects up to 10% of pregnancies, causes a surge of blood pressure, accumulation of fluid in tissues, and protein in the urine.

If untreated it can lead to eclampsia, a highly dangerous condition which produces seizures due to a lack of blood to the brain.

Magnesium sulphate treatment is widely used in the US for eclamptic conditions but much less commonly in the UK and not employed at all in many other countries.

Doctors involved in the Magpie trial believe it will swiftly lead to the treatment becoming a universal standard. Its impact in the UK may be limited since in Britain it is rare for women to develop eclampsia.

About 700 women a year are affected by pre-eclampsia in the UK but only 1% to 2% go on to develop eclampsia. The conditions kill about three or four women but up to 1,000 babies in the UK each year.

However, in developing countries eclampsia is a major killer and it is here that treatment with magnesium sulphate is likely to reap the most rewards.

Magnesium sulphate is very cheap - an entire treatment costing only about £3 or £4. Yet many underdeveloped countries have no access to the drug.

Dr Lelia Duley, a medical research council senior clinical fellow at Oxford University who led the trial, said: "Eclampsia is a devastating condition that can kill both mother and child.

"Our trial has shown that giving magnesium sulphate injections halves the risk of developing eclampsia in women who already have pre-eclampsia, and it seems likely it also reduces the risk of maternal death.

"The treatment could save countless lives across the world if it was introduced routinely for pregnant women with pre-eclampsia. And, importantly, it is a very inexpensive treatment, making it especially suitable for use in low income countries."

She suspected the lack of availability of magnesium sulphate in many parts of the world was down to the fact that it was not a money spinner.

"I think one factor is that it's too cheap for drug companies to be interested in," she said.

The trial results were published today in the Lancet medical journal. They showed that the treatment had the bonus benefit of cutting by a third the risk of placental eruption, another serious complication which can kill a baby.

Claire Giles, 32, from Heston, west London, who took part in the trial, recalled the horrific experience of her first pregnancy when she developed pre-eclampsia.

She was 32-weeks pregnant when she began to develop swelling, but at first did not realise anything was seriously wrong. "By 38 weeks I was so swollen my back was like a water bed," she told a news conference at the Medical Research Council in London. "My legs measured three foot round each. My feet were so swollen my toes were an inch off the ground."

After she was admitted to hospital doctors asked if she would be willing to join the trial. Her baby Matthew was delivered by Caesarean section on October 13 1999 and is now a healthy child.