The EU Fourth Motor Insurance Directive, which came into force on January 20, requires fleets to register details of all vehicles under their insurance that are on the fleet for longer than 14 days, or face fines of up to £5,000.
But because registration cannot be backdated, rental vehicles must be registered before they have reached the two week limit.
Fleet operators were warned of the dangers of ignoring the smallprint in the legislation at the East Anglian regional meeting of the Association of Car Fleet Operators (ACFO).
Regional chairman Julie Jenner, fleet manager of Nokia, said: 'Registering hire car vehicles that you have for more than 14 days must be done before the 14th day – you cannot do it on the 15th day and backdate it. A couple of fleets have been caught out over this.'
Jenner said she was sure there would be a settling-in period before any legal action was taken against a fleet for unwittingly not complying with the rules.
It was suggested last month that nearly half of all UK companies risk being fined because they missed the January 20 deadline to get their fleet details registered on the Motor Insurance Database.