Fleet decision-makers were challenged to take the wheel of a driving simulator that replicated driving an automatic car, with just a steering wheel, accelerator and brake pedals, and the road ahead projected on a screen.
Drivers were allowed to accustom themselves to the route - a lap of Oulton Park -before driving the circuit while simultaneously completing tasks or answering questions asked via a headset, such as 'count backwards from 150 in multiples of five' and 'describe what a STOP sign looks like'.
On average, the decision-makers were 15.7% slower when answering the questions, and spun or crashed the car an average of 2.64 times, having completed previous clear laps without incident.
The hands-free challenge was organised by risk management consultancy Andy Price Associates which developed the simulator to train fleet drivers at companies intending to implement a car phone ban.
Andy Price said: 'Those drivers who did manage to achieve a similar lap time while on the phone were either driving very cautiously or they failed to answer any of the questions asked.
'There were also a large number of drivers who could not answer any of the questions, even though when asked afterwards could easily answer them.'
He added that the experiment took place in 'perfect conditions' with no indicators, no pedestrians, and clear weather, while the real danger of using a mobile phone, whether hands-held or hands-free, occurs when a situation develops and drivers are concentrating on the call rather than the road ahead.