It can lead to cognitive decline when the mobile phone is always around.

It has long been believed that the smartphones in our pockets are actually making us dumber; but now there is evidence for it.

The constant presence of a mobile phone has a "brain drain" effect that significantly reduces people's intelligence and attention spans, a study has found.

Researchers at the University of Texas discovered that people are worse at conducting tasks and remembering information if they have a smartphone within eye shot. In two experiments they found phones sitting on a desk or even in a pocket or handbag would distract users and lead to worse test scores even when it was set up not to disturb test subjects.

The effect was measurable even when the phones were switched off, and was worse for those who were deemed more dependent on their mobiles.

"Although these devices have immense potential to improve welfare, their persistent presence may come at a cognitive cost," said Dr Adrian Ward, the lead author of the study. "Even when people are successful at maintaining sustained attention - as when avoiding the temptation to check their phones - the mere presence of these devices reduces available cognitive capability."