You must display a GB sign - failure to comply could result in an on-the-spot fine.

If your car has number plates that include the GB euro-symbol (Europlates) you don't have to display a conventional GB 'sticker' within the EU.

In some countries outside the EU a GB 'sticker' is required even if you have euro-plates, so it is always safer to display one.

The distinguishing sign for vehicles travelling in International Traffic is governed by the 1949 Road Traffic Convention, annexe 4 of which stipulates the size and appearance of the sign - one to three letters in capital latin characters with a minimum height of 80mm (3.1in.) and a stroke width of 10mm (0.4in.). The letters shall be painted in black on a white background of elliptical form with the major axis horizontal.

http://www.theaa.com/motoring_advice/overs...ral_advice.html