The Department for Transport does not hold estimates of the number of cars that are not insured or taxed.
However, the Department's latest estimates of vehicle excise duty evasion suggest that there were around 589,000 unlicensed vehicles in Great Britain in 2007.
In addition, the latest estimate of uninsured driving, based on a 2005 comparison of the DVLA vehicle register and the motor insurance database, suggests that about 2.1 million licensed vehicles are being driven by uninsured drivers.
Measures have already been introduced to tackle uninsured driving including police powers to seize vehicles used on the road uninsured and improved police access to information contained in the insurance industry's database. The Road Safety Act 2006 introduced a new offence of being a registered keeper for which there is no valid motor insurance. Further regulations are required to bring in the provisions into force and the detail of this scheme for continuous insurance enforcement will be subject to consultation later this year.