Only one in four have been clearly updated on new company policy as a result of the ban, new research by Green Flag Motoring Assistance suggests.
Since December, it has been illegal to use a hand-held mobile phone while driving. Anyone caught doing so risks a fixed penalty of £30 or, if convicted, a fine of up to £1,000, plus three points on their driving licence.
But the Green Flag survey of 700 motorists found that one in 10 are using their mobile phone handsets while driving.
Green Flag spokesman Melanie Denny said: 'The survey shows there are still a significant number of motorists prepared to risk a fine or even an accident by using a mobile phone handset when driving.
'Our research shows the majority of motorists are aware of the ban, and the dangers associated with mobile phone use, yet they are still prepared to take risks.'
Although some motorists do continue to use a hand-held mobile, more than three-quarters agreed that it severely impaired their ability to drive safely.
More encouragingly, 64% said they never use their hand-held mobile phone while driving and 28% said they had made a conscious effort to use a hands-free kit since the ban. Just 1% of those questioned were unaware of the ban.