Researchers at the University of London Institute of Psychiatry have found that the constant distractions of email and texting are more harmful to performance than cannabis.

Those distracted by incoming email, phone calls and text messages saw a 10-point fall in their IQ, more than twice that found in studies of the impact of smoking cannabis, according to the researchers.

Some 1,100 volunteers were used in the study, sponsored by HP. Half of those questioned said that they reply to emails instantly or as soon as possible, and one in five admitted to breaking off meals or social engagements to deal with email.

This constant shifting of concentration makes the brain more tired and less focused, and causes the temporary IQ fall-off.

Email and SMS are also making us work longer. Over 60 per cent of those questioned answered work emails at home or when they are on holiday.

The phenomenon of email addiction is well known, not least to users of RIM's BlackBerry devices.

Intel president Craig Barratt and many others refer to these devices as 'CrackBerrys' because of the obsessive email use they inspire in their owners.