And these drivers have on average 30 per cent slower reaction times than those who have been drinking and 50 per cent slower times than sober drivers.
The findings come about from research at the University of Sydney, Britain's Transport Research Laboratory and the University of Utah, which show that a quarter of UK drivers use their phone while driving, either with a hands-free kit, or by holding their mobile.
The findings have sparked calls for action to make drivers turn off their phone when they step into their vehicle.
According to experts, it is not the holding of the phone which is the distraction but the concentration on the conversation.
"The research indicates that the combination of visual, auditory, mental and physical distractions caused by using mobile phones can cause a severe deterioration in the ability of the driver to react quickly to changing conditions," the Daily Mail quoted Paul Purdy of yesinsurance.co.uk, as saying.
"We would like to see the launch of a campaign that urges drivers to switch off their mobile phone before setting off on a trip, so that messages can be received by voicemail," he added.
According to research conducted by the Transport Research Laboratory, at any one time 2.5 per cent of drivers are using the phone while driving - with two-thirds illegally using handheld phones.
"While we would not go as far as calling for a complete ban on hands-free devices, we would like to see the Government placing a much greater emphasis on warning drivers that using a hands-free device can be as dangerous as drinking," Purdy said.
"Conversations with passengers can incorporate periods of silence but telephone conversations tend to be continuous, requiring a greater commitment in terms of concentration from the driver," he added.