Pregnant women can safely drink coffee as caffeine does not affect their unborn baby, according to a new study.

Researchers found no evidence of a link between prematurity, birth weight and the amount of caffeine consumed by motherstobe.

Previous studies suggested caffeine might harm unborn babies as it stays in the system longer in pregnant women, passing easily to a growing baby.

Health officials have warned that a high caffeine intake could affect birth weight or the chance of having a miscarriage.

Pregnant women are advised against drinking more than four cups of coffee a day - or six cups of tea.

The study, by the University of Aarhus in Denmark, recruited more than 1,000 women before they were 20 weeks' pregnant, who drank at least three cups of coffee a day.

The group was split into two, with 568 women drinking ordinary instant coffee and 629 drinking decaffeinated.

Each woman was regularly monitored to check her caffeine intake, including from drinks such as cola.

The authors then monitored the birth weight of 1,150 newborn babies and the length of pregnancy for 1,153 of the babies.

The study, published online today by the British Medical Journal, found 'no significant differences' between the two groups for birth weight or length of pregnancy.

The researchers concluded that a moderate reduction in caffeine intake in the second half of pregnancy had 'no effect' on the outcome.

The women were not told what type of coffee they were drinking, and the research was adjusted to take into account factors such as age, weight and whether the women smoked.

When the adjustments were made, the average weight of babies born to women in the decaffeinated group was a mere 16g higher than those born to women in the caffeinated group, the study said.

The average difference in the length of pregnancy was less than two days.

A spokesman for the British Coffee Association said: "This new study is very interesting and supports the consistent advice given that pregnant women should stick to a safe upper limit - in line with guidance issued by the Food Standards Agency. This equates to three cups of brewed, or four cups of instant coffee.

"Coffee is one of the most heavily researched commodities in the world today and this adds to the wealth of scientific evidence which suggests that moderate coffee consumption is perfectly safe."