A motoring organisation has claimed "big questions remain" about the recall of one in 10 diesel cars in the UK over the Volkswagen emissions scandal.
Over 1.1 million cars in the UK have been fitted with software used to con emissions testers in the US, Volkswagen said.
It announced plans to contact owners to arrange for their vehicles to be "corrected".
But motoring research charity, the RAC Foundation, said drivers are concerned about what effect the software has had on their cars, and what impact the recall will have on performance.
The organisation's director, Steve Gooding, commented: "Slowly but surely the true impact of this deceit is being revealed. But big questions remain, not least why do VW Group cars in the UK need to be recalled at all?
"If the defeat technology was installed but not used what is the point of the work? Is there still more to this than we are being told?
"We all want clean air to breathe, but motorists will understandably be worried about the MPG (miles per gallon) implications of these so-called corrections on the cars many will have bought on the strength of their fuel economy."
VW confirmed that the affected vehicles in the UK consist of 508,276 Volkswagen cars, 393,450 Audis, 131,569 Skodas, 79,838 VW commercial vehicles and 76,773 Seats.
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 has admitted that 11 million of its diesel vehicles worldwide were fitted with the software.
The company said that just over 1.1 million UK cars are affected, which is 9.9% of the country's diesel cars.
Meanwhile several manufacturers have hit back at a study by German-based motoring organisation, Adac, which found that high levels of pollution are being emitted by new diesel vehicles built by a range of car makers other than VW.
Nissan, the UK's biggest car maker, said its model used in the test had an older Euro 5 engine rather than a new one compliant to Euro 6 emissions regulations.
A Renault Group spokesman said: "The group complies with all regulations and legislation for the markets in which it operates. Its vehicles are not equipped with defeat devices."
Hyundai issued a statement which read: "Hyundai Motor abides by the testing regulations and methods of each region where it sells cars including Europe."
Mike Hawes, chief executive of the Society of Motor Manufacturers and Traders, admitted that motorists "are right to be concerned" by VW's actions but said "the actions of one company do not mean collusion".
He added: "It would be wrong to penalise all diesels. The latest diesel vehicles are the cleanest ever... effectively reducing nitrogen oxide levels by 92% compared with earlier generations."