Fleets which have not yet submitted the information must do so now if they are to meet the directive's requirements from Monday's deadline. It also requires fleet operators to keep the information up to date as the fleet changes.
This week, the Government revealed its much-delayed explanation of how the system will work. It includes demands for insurers to keep records of vehicles covered going back seven years.
For short-term vehicles not required to be listed, such as rental cars under open cover policies for less than 15 days, fleets will be required to keep records for the same length of time.
The MID was created and is managed by the Motor Insurers' Information Centre (MIIC) to reduce the problem of uninsured driving which it says costs UK motorists about £600 million a year.
Executives say they have given a number of presentations to fleet operators and trade associations on the actions they need to take to comply with the new law. It adds that this information has been passed on to insurers and brokers who have taken their own steps to communicate the new legislation with policyholders.
But at a recent conference hosted by Hertz, delegates heard that insurers and fleets were still confused. Association of Car Fleet Operators director Stewart Whyte, who spoke at the event, warned: 'This Directive will mean a monumental change for fleets, particularly to their archiving systems.'
Last month MIIC programme manager Donald Martin said fleet operators who do not supply their vehicle data by the deadline risk 'penalties enforced by the law'.
He added: 'For those who have not yet begun preparations to supply data, remaining in ignorance is not an option.'
Martin said fleets should have contacted their insurance company or broker to find out about loading their vehicle insurance data.