All diesel cars tested by the Government after the Volkswagen emissions scandal exceed laboratory pollution limits in real-world conditions, a study says.
A report by the Department for Transport on nitrogen oxide levels revealed that none of the 37 top-selling diesel vehicles met the legal level of 180mg/km when observed out on the road.
But a spokesman for motor industry body the Society of Motor Manufacturers and Traders said the difference between results from laboratory tests and those performed in the real world are "well known".
Cars that met Euro 5 standards, which could be sold up to September last year, were all "substantially higher" in real-world conditions than the measurements recorded in the laboratory, according to the research.
The worst performer was the Vauxhall Insignia, which emitted more than 1,800 mg/km.
Even the best performer, the Citroen C4, was found to emit around three times the legal laboratory level.
Real-world emissions tests will be introduced next year, although diesel cars will at first be allowed to pollute more than double the current legal level because of their limited ability to reduce real-world emissions in the short term.
Cars that meet the current Euro 6 standards have to meet 80 mg/km NOx limits in laboratory tests, but in real-world driving for 90 minutes on normal roads the average level recorded was six times higher.
The Peugeot 3008 was found to perform the worst, at around 13 times higher.
The £1m investigation found no evidence of car manufacturers other than Volkswagen fitting devices to cheat emissions tests.
But the study did reveal the widespread use of systems to prevent engine damage, which can also lead to higher emissions in the real-world when the temperature is lower than during the approved laboratory test.
VW admitted last September that 482,000 of its diesel vehicles in the US were fitted with defeat device software to switch engines to a cleaner mode when they were being tested for emissions.
The German-based firm said some 11 million vehicles were affected worldwide - including almost 1.2 million in the UK.
Transport Secretary Patrick McLoughlin said: "Our tests published today have not detected evidence of manipulation of emissions lab tests as used by the VW Group by any other car manufacturer."
He added: "The UK has been leading in Europe in pushing for real-world emissions tests which will address this problem.
"Real-world tests will be introduced next year to reduce harmful emissions, improve air quality and give consumers confidence in the performance of their cars.
"Following the Volkswagen emissions scandal, the whole of the automotive industry must work hard to restore public trust by being transparent about the systems they employ and advancing plans for introducing cleaner engine technology."
Transport Minister Robert Goodwill insisted the manufacturers "have not done anything illegal" by using engine management systems which cut emissions outside of the laboratory.
On the real-world emissions observed, he said: "What's been disappointing is the levels of non-compliance have been higher than we'd expected."
Mr Goodwill continued: "It's a bit like, you can pass your driving test on the day that you're being very careful but then in the real world it's slightly different.
"That's the difficulty that we face."