The official name for the GB sticker is Distinguishing Sign of Vehicles in International Traffic, and it is covered by the 1968 Vienna Convention, Annex 4. When travelling abroad, it is a legal requirement to displays the correct sign.
The main points concerning the design of GB stickers stipulate that only black font on white background can be used, the minimum height of the font is 80mm, and the minimum size of the eilipse-shaped sign is 175mm by 215mm (4.5″ by 6.9″).
Annex 4 of 1968 Vienna Convention stipulates the following:
1. The distinguishing sign shall be composed of one to three letters in capital Latin characters. The letters shall have a minimum height of 80mm (3.1 in.) and their strokes a width of 10 mm (0.4 in.) The letters shall be painted in black on a white background of elliptical form with the major axis horizontal.
2. If the distinguishing sign is composed of three letters, the dimensions of she ellipse shall be at least 240 mm (9.4 in.) in width and 145 mm (5.7 in.) in height. The dimensions may be reduced to 215 mm (6.9 in.) in width and 175 mm (4.5 in.) in height if the sign carries less than three letters.
The requirement to display a separate distinguishing sign is not necessary within the European Economic Area for vehicles with license plates in the common EU format which satisfy the requirements of the Vienna Convention.
Therefore, following Brexit, British drivers will not be able to rely on their EU-format licence plate, and will have to display the correct GB sticker.