The RAC said the decision to get rid of the paper tax disc three years ago has proved "costly" when it should have saved the Treasury £10m a year.
"It appears that having a visual reminder was an effective way to prompt drivers into renewing their car tax - arguably more drivers are now prepared to try their luck and see if they can get away with not paying any vehicle tax at all, or are simply forgetting to tax their vehicle when they are due to."
When the abolition of the paper tax disc was announced by then-Chancellor, George Osborne, the Treasury said it showed government was moving "into the modern age".
The RAC said a third of untaxed vehicles had changed hands since September 2016, indicating that many drivers were not aware that tax does not carry over when ownership changes.
The seller receives a refund of any full months of remaining tax while the new owner must tax the vehicle immediately.