Motorists will not be forced to pay more in car tax even if their vehicles are found to be fitted with illegal software to cheat emissions tests, the Government has confirmed.
German car-maker Volkswagen has admitted that 11 million of its diesel vehicles worldwide are fitted with defeat device software.
The UK Government has launched an investigation to re-test cars to compare their laboratory results with real-world driving emissions
Transport Secretary Patrick McLoughlin said: "Our priority is to protect the public and give them full confidence in diesel tests.
"The Government expects VW to support owners of these vehicles already purchased in the UK and we are playing our part by ensuring no-one will end up with higher tax costs as a result of this scandal."
The amount of vehicle excise duty - commonly known as car tax - depends on a vehicle's CO2 emissions.
Mr McLoughlin went on: "We are... starting our testing programme to get to the bottom of what the situation is for, VW cars in the UK, and understand the wider implications for other car types to give all consumers certainty.
"I have been pressing for action at an EU-level to improve emissions tests and will continue to do so. I have also called for a Europe-wide investigation into the use of defeat devices, in parallel to the work we are doing in the UK."
The Department for Transport said its inquiry will involve cars and testing facilities that are not provided by the motor industry.
UK officials are liaising with authorities in other EU countries who are conducting similar investigations.
It is hoped that this coordinated approach will improve consistency and enable the widest range of vehicles to be tested across Europe.
AA president Edmund King described the clarification on car tax as "common sense".
He said: "When customers bought these cars it was in good faith. They bought them based on the details that were available at the time.
"To add to their tax would be grossly unfair.
"This is a common sense and pragmatic approach and it will put many drivers' minds at ease."
The scandal began in the US, where the Environmental Protection Agency said 482,000 Volkswagen vehicles were fitted with sophisticated defeat device software, which switches engines to a cleaner mode when they are undergoing official testing.
Once on the road the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.
VW is contacting 1.2 million motorists in the UK to arrange for their vehicles to be "corrected".
The Wolfsburg-based firm has also taken 4,000 vehicles across the Volkswagen, Audi, Skoda and Seat brands which have the EA 189 engine off the market.
VW said this equates to 3% of new vehicle stock.
The decision was taken to enable the firm to "resolve the current issue with the vehicles", a spokesman said.
Mike Hawes, chief executive of the Society of Motor Manufacturers and Traders, admitted that motorists "are right to be concerned" by VW use of illegal software but claimed "the actions of one company do not mean collusion".