New doubts have been cast on the safety of hands-free mobile phones after a study by the Transport Research Laboratory (TRL) showed that drivers using them have slower reaction times and are more likely to miss potential hazards.

Van drivers wishing to use mobile phones on the move have been forced down the hands-free route after it became illegal to use hand-held units.

This offence now attracts a £60 fine and three licence penalty points.

But the report said it was wrong to assume that hands-free kits were safe and urged Britain’s van fleet operators to instruct staff not to use any phone while on the move.

In the study, 30 volunteers drove an 11-mile route in a simulator.

Those who had conversations using hands-free kits were more likely to miss road signs and an analysis of their conversations showed that their ability to communicate was poorer than when talking to a passenger.

The study concluded: ‘Driving and talking is cognitively demanding and results in poorer driving performance.

Talking on a hands-free phone while driving can result in poorer decision- making.’

TRL said it may be necessary in the future to ban hands-free phone conversations on the move too.

The report was welcomed by the Royal Society for the Prevention of Accidents.

A spokesman said: ‘There is now plenty of evidence to show that it is the conversation itself, not the type of phone, that causes the distraction.’