The latest research will fuel concern about the safety of mobile phones, which have already been blamed for causing brain tumours, headaches and fatigue.

Doctors at the University Neurology Clinic in Freiburg, Germany, carried out tests by strapping mobile phones to the heads of ten young volunteers. They found that blood pressure increased after 35 minutes of exposure to the mobile phones. Although the rise was small, doctors said it could be harmful to people who already suffer from high blood pressure.

Dr. Duncan Dymond, consultant cardiologist at St Bartholomew's Hospital in London, told the Mail: 'A sudden surge in blood pressure can cause problems, especially for people with coronary artery disease.'

Alasdair Phillips of Powerwatch, a consumer service which provides information on electro-magnetic fields, said mobile phone sellers should be required to put labels on their products warning against extensive use. He told the Mail: 'The study could provide a medical explanation for the three to six-fold increase in headaches reported among those using their phones for more than an hour a day. 'Mobile phones should be used for essential communications only and calls should be kept very short.'

The World Health Organisation has launched an investigation into whether mobile phones can cause brain tumours.

Recent Swedish research also found a dramatic increase in headaches and fatigue among people using mobile phones.