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."

The researchers tested 520 university students on their memory and intelligence when in the presence of a smartphone to see how it affected them.

Participants were told to complete tests in mathematics, memory and reasoning with their smartphones either on their desk, in their bag or pockets, or in another room, and with alerts turned off so as not to distract students.

Those who had their phones on the desk recorded a 10 percent lower score than those who left them in a different room on operational span tasks, which measures working memory and focus. Those who kept their phones further out of sight in their pockets or their bags scored only slightly better than when phones were placed on desks.

The researchers found that the negative effect of having a phone within eyeshot was significantly greater among those who said they were dependent on their smartphones. Participants who had expressed sympathy with phrases such as "I would have trouble getting through a normal day without my cellphone" and “using my cellphone makes me feel happy” performed as well as others when their phone was in a different room, but worse when it was placed on their desk.

The study also found reaction speeds to be affected, with students who had their phone on the desk responding more sluggishly in high-pace tests.

It even found that phones can even distract users even when they are turned off and placed face down. Those with phones outside of the room "slightly outperformed" those with switched off devices.

The researchers said the effect arises because part of a smartphone users' mind is dedicated to trying to not think about distractions such as whether they have any messages when the handset is in their line of sight.

"We see a linear trend that suggests that as the smartphone becomes more noticeable, participants' available cognitive capacity decreases," said Ward. "Your conscious mind isn't thinking about your smartphone, but that process - the process of requiring yourself not to think about something - uses up some of your limited cognitive resources. It's a brain drain."

Similar research has previously showed smartphones can have a "butterfly brain effect" on users that can cause mental blunders.