If you see an untaxed vehicle on a public road, you can report it. Your report is anonymous and will be investigated.
There is a different way to report:
Steps to report a vehicle as untaxed
Check if the vehicle is taxed before you report it - this is a free service.
Make a note of the vehicle registration number, make, model and colour of the vehicle and the street name, town and postcode where the vehicle is parked. You will not be able to report the vehicle without these details.
Further information on reporting an untaxed vehicle can be found on GOV.UK's website.