The change has been small, but possibly it could be as much as six months

Prof Peter Whincup

Professor Peter Whincup, who carried out the survey, said: "The change has been small, but possibly it could be as much as six months, although that's unlikely.

"It is difficult to say whether it will fall further. At the moment it does seem to be pretty stable.

"But one thing we did find was differences of provision for girls in primary schools and I think this needs to be taken on board."

Over 1,000 girls aged 12-16 in schools across 10 British towns took part in the survey.

Better communication

The Family Planning Association (FPA) says the findings illustrate the need for better communication on sexual health matters so that children are prepared for the way their bodies will change before the changes materialise.

FPA Director of Information Toni Belfield said: "We have always said that when a young woman asks a question, you need to answer it in the most appropriate way, both at home and at school.

"Young women need support and information about the changes they will see in their bodies as they get older."

The most significant changes in menarche happened in European countries (including Britain) between the late 19th and mid 20th centuries, when the average age fell by more than a year.