Volkswagen's admission that some of its cars cheated clean-air tests in the US has led to calls for stricter testing in Europe.

The German car-maker said 11 million vehicles worldwide might have been fitted with software to trick testers into believing they met environmental standards.

It is not known whether cars on Britain's roads are affected - or how many of them - but campaigners have demanded that the process for testing vehicles is made more stringent.

Tim Barlow, air quality expert at the Transport Research Laboratory, said: "Current testing methods are outdated and offer room for error or optimisation, so it's imperative that industry, governments and regulatory bodies work together to find the best way forward.

"Ideally we need to move towards a testing model that's based on real driving emissions, carried out with vehicles operated on normal roads.

"This should be followed up with in-use compliance testing, whereby a sample of vehicles already in use are tested to check they still comply with the emissions limits."

The Environmental Protection Agency (EPA) in the US said cars had been fitted with sophisticated software to switch engines to a cleaner mode when they are undergoing official emissions testing.

This 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.

Monique Goyens, director general of European consumer organisation the BEUC, said real-life testing was urgently needed on this side of the Atlantic too.

"We've been saying long before this scandal broke out that one of the problems in the EU, unlike in the US, is the absence of a surveillance system which would require independent on-the-road testing," she said

"The EU needs to implement such a system to restore trust amongst consumers in emissions and fuel consumption test programmes."

Details of a real driving emissions test incorporating modern technologies and on-road conditions are being discussed by the industry and the European Commission.

It has been proposed that the test is introduced in 2017, but UK Transport Secretary Patrick McLoughlin has written to the European Commission to call for it to be introduced as soon as possible in the wake of the VW scandal.

The Society of Motor Manufacturers and Traders (SMMT) issued a statement which said the industry "accepts that the current test method for cars is out of date" and is seeking agreement with the European Commission for a process that "embraces new testing technologies and which is more representative of on-road conditions".