Millions of cars on UK roads could be recalled as a probe into rigged emissions tests on Volkswagen models in the US threatened to reach Europe, according to a transport lobby group.
The German car maker apologised after America's Environmental Protection Agency (EPA) found the company had cheated clean-air rules before ordering it to recall nearly half a million diesel models built in the last seven years.
The US government announced it was widening the investigation to other manufacturers, while the European Commission (EC) has contacted VW and US authorities over the findings.
Campaign group Transport & Environment (T&E) claimed the technology used in the VW cars, in the form of devices that allowed its diesel cars to release fewer smog-causing pollutants during tests than in real-world driving conditions, was used by other car makers.
Greg Archer, T&E's clean vehicles manager, told the Daily Telegraph: "The Volkswagen example is clearly just the tip of the iceberg and there will be a lot more companies dragged into this.
"Exactly the same technology is used in Europe as it is in the US, and tens of millions of cars have been sold since 2009. That mean that there are potentially millions of cars that are being driven illegally.
"Obviously it is not the drivers' fault, but it is likely we will see a huge number of recalls."
Diesel car sales make up almost 50% of the total sold in the UK this year, according to the latest figures from the Society of Motor Manufacturers and Traders.
The EPA's findings cover 482,000 cars including the VW-manufactured Audi A3, and the brand's own Jetta, Beetle, Golf and Passat models.
The agency said cars had been fitted with sophisticated software algorithms which detect when they are undergoing officials emissions testing, and turns full emissions controls on only reducing the test. It is a type of software known as a "defeat device".
Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.
With the world's top-selling car maker facing billions of dollars in fines and a plunge in share prices, VW chief executive Martin Winterkorn said: "I personally am deeply sorry that we have broken the trust of our customers and the public.
"We will co-operate fully with the responsible agencies, with transparency and urgency, to clearly, openly, and completely establish all of the facts of this case. Volkswagen has ordered an external investigation of this matter.
"We do not and will not tolerate violations of any kind of our internal rules or of the law."
An EC spokesman told German broadcaster Deutsche Welle: "It is premature to comment on whether any specific immediate surveillance measures are also necessary in Europe and whether vehicles sold by Volkswagen in Europe are also affected. We are taking the matter very seriously."
The EU recently adopted new procedures to test real-world emissions under the Euro 6 scheme.