Motorists wanting to cross the Irish border will need a "Green Card" in the event of a no-deal Brexit.
Insurers have warned motorists to arrange this extra documentation if the UK crashes out of the EU on 29th March.
This also applies to millions of other UK drivers and companies wanting to drive in EU countries.
It's been recommended to obtain a Green Card from your provider a month before you plan to travel.
By driving without a Green Card in the event of a no-deal Brexit, you risk breaking the law.
Rules apply both ways
The same rules apply to European Economic Area (EEA) motorists travelling to the UK.
Huw Evans, director general of the Association of British Insurers, said: "As it looks increasingly possible that a 'no-deal' Brexit may happen, we want all insurance customers to know the facts about what this means for them.
Mr Evans warned: "You should contact your insurer before you travel in order to get one. This advice applies to businesses as well as individuals."
Waive the need for Green Cards
An agreement between UK and European insurance authorities was struck in May 2018 to waive the need for Green Cards in the event of a no-deal Brexit.
However the agreement has not yet been ratified by the European Commission.
The Department of Transport issued guidance to motorists and insurers in September.
It said that the cards would be issued free of charge, although insurers may increase their administrations fees to reflect the cost of providing them.
Commercial operators with fleet insurance were advised they will need a card for each vehicle.
For countries that require separate trailer insurance, a separate Green Card may be required for the trailer.
Without a Green Card, motorists would have to be covered by third-party insurance bought in the country they are driving in.
Without it drivers may not be able to drive and could also be fined.
The ABI said it is against the UK leaving the EU without a deal, with Mr Evans warning such an outcome would be "bad for the economy and bad for our customers".