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 Europe's laboratory system, which dates back to 1996, needs bringing up to date.
He said: "Current testing methods are outdated and offer room for error or optimisation, so it's imperative that industry, governments and regulatory bodies work together to find the best way forward.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.
He wrote: "The UK is committed to delivering cleaner air for our citizens and a key element of this is setting the correct standards for new cars at EU level.
"The UK has pressed for the earliest introduction of the new real driving emissions test and the current issue reinforces my view that we must finalise these measures urgently and so set a clear obligation for vehicle manufacturers."
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".
The SMMT said cars sold in the UK "must comply with strict European laws".
VW's chief executive, Martin Winterkorn, is under intense pressure over his job as the firm holds an emergency board meeting today.
The Wolfsburg-based firm will need to formulate a co-ordinated response as it faces deepening scrutiny.
New York attorney general Eric Schneiderman said he would collaborate with other US states to enforce consumer and environmental law.
Mr Schneiderman said: "No company should be allowed to evade our environmental laws or promise consumers a fake bill of goods."
He added: "We look forward to collaborating with attorneys general across the nation on this matter."
Other states are also looking at filing class actions suits against the world's biggest car-maker and, according to reports the US Department of Justice (DoJ) is looking into the issue, which raises the possibility of the company and individual executives facing criminal charges.
In the past the DoJ has often extracted hefty payments from companies to settle criminal charges.
Meanwhile, the EPA and the California Air Resources Board are investigating the way VW cheated tests with its diesel cars.
Shares in the firm were up almost 3% in early trading, but since the scandal broke at the start of the week the firm has lost around a third of its value, or 26 billion euros (£19 billion).
Stocks in rival European car-makers such as Germany's Daimler and BMW have also fallen this week, but made gains of just under 2% in early trading today.
Mr Winterkorn said he was ''endlessly sorry'' over the scandal, which has prompted questions about whether he can remain at the business.
VW's shares are at around 107 euros (£78), and have already had a tough year, having fallen from more than 250 euros (£180) amid signs of faltering sales in the US and China.