The average fine given to uninsured drivers has fallen by more than a fifth since Labour came to power 11 years ago.
Those convicted of driving while uninsured were fined an average of 177 in 2005, compared with 224 in 1997. The number of uninsured motorists appearing in court fell in the same period.
The latest Government figures estimate that there are more than two million uninsured vehicles on the road.
A Conservative spokesman, David Ruffley, blamed the fall in prosecution on police being bound up by red tape and bureaucracy.