It said it feared that the number of tax-dodgers could equal the number who try to avoid paying motor insurance.
RAC chief engineer David Bizley said: “We could be looking at around £167m of lost revenues to the Treasury, far exceeding the £10m that will be saved by no longer having to print tax discs and post them to vehicle owners.”
From October 1, motorists will no longer need to display a tax disc on their vehicle windscreen. They will still need to pay their vehicle excise duty car tax, with records being monitored electronically.
An RAC survey of more than 2,000 drivers showed that 36 per cent were unaware of the scrapping of the paper disc, while 47 per cent did not know when the change was due to take effect.
The poll also showed 63 per cent feared there would be a rise in the number of untaxed cars on the road, while 44 per cent reckoned the change would encourage people to break the law.
From October 1 vehicle tax will no longer be transferred when a vehicle is sold, with buyers of used vehicles having to renew the tax when they make a purchase.
Mr Bizley said: “There is clearly concern among motorists over the issue of enforcement. Most of the changes make sense and will benefit the motorist, but too many motorists are unaware of the detail.
“The big question has to be whether enforcement using only cameras and automatic number plate recognition will be sufficient.”
A Driver and Vehicle Licensing Agency spokesman said: “There is no basis to these figures and it is nonsense to suggest that getting rid of the tax disc will lead to an increase in vehicle tax evasion.”
“We have a proven track record in making vehicle tax easy to pay but hard to avoid, with over 99% of all vehicles taxed. Given the systems now in place we take enforcement action direct from our electronic records rather than requiring a tax disc.”