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From http://www.dvla.gov.uk/drivers/rdmcycle.htm

4. The New Category A and A1 Driving Licence

At age 17 or over, you will have two types of full motorcycle licence to aim for - the A1 light motorcycle licence or the standard category A motorcycle licence.

To gain a full A1 light motorcycle licence you must successfully complete CBT and pass a theory test followed by a practical test on a vehicle between 75cc and 125cc. A full A1 licence permits you to ride any motorcycle up to 125cc and a power output of up to 11kW (14.6 bhp) without 'L' plates, carry pillion passengers and use motorways.

To gain a full standard category A licence you must successfully complete CBT and pass a theory test followed by a practical test on a motorcycle of over 120cc but not larger than 125cc and capable of at least 100kph. In practice the normal test vehicle will be a 125cc machine. A full standard category A licence permits you ride any motorcycle with a power output of up to 25kW (33bhp) and a power-to-weight ratio not exceeding 0.16kW/kg without 'L' plates, carry pillion passengers and use motorways. You are restricted to a motorcycle up to 25kW for two years (not counting any periods of disqualification). After two years you may ride any size motorcycle.