Motorcycles registered on or after 1 September 2001 must display a two-line number plate. Motorcycles registered prior to the 1 September 2001 may display a three-line plate, but one-line plates are illegal, irrespective of the date of registration of the motorcycle or the size of the letters. This is not an endorseable offence and will only involve a fine, normally a fixed penalty. Some forces are not issuing fixed penalties but are issuing a summons, in which case the fine will be higher than the fixed penalty.
Some courts are now stating that if you are prosecuted 3 times for a small/illegal number plate they will take your registration number away from you and issue a Q plate, effectively wiping out the value of your bike. The characters must be as follows: Height: 64mm, Width: 44mm. They must be 10mm apart and have a margin of 11mm around the outside and 13mm between vertical lines.