A survey by fleet management group FleetLine found that 70% of small to medium-sized enterprises (SMEs) have introduced the blanket ban despite the new legislation, which came into force this week, only banning hand-held phones.
David Harnett, FleetLine's director, said: 'It is clear that many of the businesses feel that rather than facing the possibility of potentially crippling corporate liability claims in the future, imposing a blanket ban will avoid any confusion over grey areas of the legislation.'
Parent company LeasePlan also claims fleets must provide clear guidelines or face corporate liability charges.
Kevin McNally, managing director of LeasePlan, said: 'To help protect against corporate liability claims, companies should provide explicit guidance to their drivers.'
The police also said this week that motorists will only get verbal warnings until February, when the fines will be enforced.
A number of companies are still unaware that they face hefty fines if one of their fleet drivers is caught using a mobile phone while driving. If a company 'causes or permits' one of its drivers to take a call using a mobile while on the move, then the company could be fined up to £1,000.
But research from the RAC has indicated that although the vast majority of fleets have introduced phone policies for drivers, up to 40% do not have rules regarding companies causing drivers to take a call.
Motoring groups have urged fleets to provide clear guidelines on what is acceptable in a bid to quash uncertainties over the law.