Holidaymakers and businesses intending to use their vehicles on the continent, or anyone crossing the Irish border by road, have been advised they will need a “Green Card” if the UK crashes out of the EU on March 29.
Private motorists and companies have been recommended to contact their providers around a month before they plan to travel to get one - or risk breaking the law.
The same rules apply to European Economic Area (EEA) motorists travelling to the UK.
Huw Evans, director general of the Association of British Insurers (ABI), 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.
“If you live in Northern Ireland and drive to the Republic of Ireland, or if you plan to drive your vehicle to mainland Europe after a no-deal Brexit, you will need a Green Card to prove you are insured.
“You should contact your insurer before you travel in order to get one. This advice applies to businesses as well as individuals.”
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.
There is currently a Green Card-free circulation area covering the EEA and Andorra, Serbia and Switzerland.
Systematic border checks of the document as proof of third-party motor insurance have been abolished in the area.
However, in the event of no-deal this would cease to apply for UK drivers.
The Department of Transport (DfT) 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”.
“We continue to hope these arrangements are never needed and urge the Government, UK Parliament and EU27 to agree an orderly way forward,” he said.