Oliver Letwin will today use a speech to the think tank Politeia to claim that the police's desire to meet their targets has meant they are more likely to pursue "easy crimes" such as motoring while other criminals are let off the hook.

"Picking on the easy case is a great way of raising revenue," the shadow home secretary will say.

"One need only think of the spread of speed cameras and the introduction of congestion charges. The motorist, that is the legally registered fully insured motorist driving his own vehicle, is the ultimate easy case...Meanwhile the joy-riders travel free of charge, free of restrictions and free of parking tickets."