The present maximum authorised mass (MAM) limit for category B driving licences is 3,500kg. This limit should be raised to 4,000kg. The relevant regulation would need to be changed.
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Motorhomes (motor caravans) fall into two main categories - category B for vehicle with a MAM up to 3,500kg and category C1 for a MAM up to 7,500kg. Many of these vehicles today are close to the 3,500kg limit and a lot are in the C1 category.
C1 was intended as a licence category for commercial vehicles, or small lorries. Motorhomes have fallen into this category be default because their MAM has gone above 3,500kg.