Using a smartphone to engage in social networking while behind the wheel is more dangerous than being drunk or high on drugs, a new study published by the Institute of Advanced Motorists (IAM) reveals.

The revelation is based on a study by the Transport Research Laboratory, which used a driving simulator to look at the effect of motorists using Facebook on their phones while on the move.

It found that drivers' reactions slowed by 38% on average when engaging in social networking on their phones, that they were generally unable to stay in a central lane position, and couldn't react to the gradually changing speed of the car in front.

By comparison, using a hands-free phone slows reaction times by about a quarter - more than using cannabis, which reduces them by a fifth.

Alcohol in the bloodstream at the legal limit slows reactions by 12.5% - compare that to the 38% reduction in reaction times when using social networking.

It's shocking, therefore, that 8% of drivers admit to using their smartphones while behind the wheel - as many as 3.5 million in the UK.

And, unsurprisingly, among younger drivers (17-24 years old) that proportion increases: 24% admit to emailing and using social networking sites on their phones while driving.

The IAM's Simon Best said: "This research shows how incredibly dangerous using smartphones while driving is, yet unbelievably it is a relatively common practice.

"If you're taking your hand off the wheel to use the phone, reading the phone display and thinking about your messages, then you're simply not concentrating on driving. It's antisocial networking and it's more dangerous than drink driving and it must become just as socially unacceptable."

The IAM is therefore calling for the Government to take action to highlight the dangers of phone use behind the wheel.