High street coffee shops could pose a risk to pregnant women due to large variations in caffeine content, according to research.

Analysis of espresso coffees from 20 shops found one with a caffeine content six times higher than another.

Researchers from the University of Glasgow said customers were completely unaware of the caffeine levels in cups of coffee.

The Food Standards Agency (FSA) advise that pregnant women should restrict their average daily caffeine content to 200mg.

But while a cup of coffee from Starbucks was found to have 51mg of caffeine, a Costa coffee had 157mg.

The highest amount of caffeine was found in a cup from Glasgow-based Patisserie Francoise, which was serving 322mg in a single drink.

The FSA warn that too much caffeine could result in miscarriage or a baby having a lower birth weight, which can increase the risk of some health conditions in later life.

Alan Crozier, from the research team, said: "Despite the increasing number of coffee shops on the high street and in airports, there appear to be no recent publications on the caffeine contents of the various types of commercially prepared coffees.

"This snap-shot of high street espresso coffees suggests the published assumption that a cup of strong coffee contains 50mg of caffeine may be misleading."

Researchers analysed coffees ranging in cup size from 23-70ml. Their findings are published in the latest edition of the Royal Society of Chemistry's Food and Function journal.