UK-registered cars will need to display a GB sticker in the Republic of Ireland after Brexit, the government has said.
New government advice said the sticker must be displayed in any EU country.
Motorists from the UK driving in the Republic are currently advised to display the sticker, but the rule is not widely enforced.
The advice applies to cars registered in all parts of the UK, including Northern Ireland.
The sticker, a white oval containing the letters GB, standing for Great Britain, must be displayed at the rear of the vehicle.
Northern Ireland is not a part of Great Britain - which is made up of England, Scotland and Wales - but the GB sticker is used for cars from all parts of the UK.
The rule will apply to drivers even if their number plate includes a GB logo.
Not displaying a GB sticker would not invalidate your motor insurance but motorists are urged to comply with relevant requirements, the Association of British Insurers said.
The Department for Infrastructure said the requirement that all UK motorists driving in the Republic of Ireland should display a GB sticker stems from the 1949 Geneva Convention on Road Traffic.
"It is not an EU requirement and is not affected by Brexit," the department said.
"We are not aware of any occasion when this has been enforced by the Irish government."
Sinn Féin MP for West Belfast, Paul Maskey, said he would not be displaying one of the stickers on his car.
Many nationalists in Northern Ireland, who identify as Irish rather than British, raised objections on social media to being required to have a GB sticker.
Seamus Leheny of the Freight Transport Association criticised the advice as "frivolous".
Currently in NI we have approx 1.19 million vehicles registered.— Seamus Leheny (@Freight_NI) September 2, 2019
144,248 of these are Goods Vehicles + 21,000 HGV Trailers 🚛🚚
That’s around 165k Goods Vehicles & Trailers that would have to adhere to any new frivolous cross border rules and red tape...utter nonsense #brexit pic.twitter.com/zbWC1wpUIU
The AA said that drivers from the Republic of Ireland are currently required to display Irish identification on their vehicles when travelling to all parts of the UK, including Northern Ireland.
Irish registration plates which incorporate the IRL/EU symbol are acceptable, instead of a sticker.
Owners of Irish-registered vehicles will not need a motor insurance green card to drive in the UK in the event of a no-deal Brexit.
Green cards serve as proof of insurance.
The Motor Insurers' Bureau of Ireland said valid Irish insurance discs would be accepted as proof of insurance for Irish vehicles.
In a no-deal scenario though, UK drivers may still need a green card to drive in the EU, including the Republic.