Dozens of new models of diesel cars emit harmful nitrogen oxide pollution that exceeds the latest pollution standards, campaigners warn.
Results from "on-road" test results for new Euro 6 diesel models approved for sale in the months before new pollution limits came into force showed half were more polluting than would soon be allowed, Greenpeace said.
The environmental group claimed the results undermined industry claims that new diesels were the "cleanest in history".
But the Society of Motor Manufacturers and Traders (SMMT), which represents the UK automotive industry, said it was "false and inaccurate" to claim that car makers were breaching emissions rules.
Tougher on-road tests were introduced in the wake of the 2015 dieselgate scandal over Volkswagen installing "defeat device" software to cheat emissions tests in the lab.
New nitrogen oxide limits reflecting the on-road tests did not come into force until September 2017, and were around double the previous maximum levels for Euro 6 vehicles based on lab testing.
Unearthed, Greenpeace's investigative news arm, obtained and analysed a data detailing the nitrogen oxide emissions from on-road tests of cars approved for sale between April 2016 and September 2017.
It suggests 48 out of 94 diesel models from big selling European brands were above the limits brought in shortly afterwards, and two-thirds were well above the original Euro 6 limits based on lab tests.
Paul Morozzo, Greenpeace campaigner, said: "Carmakers, backed by the industry lobby, are yet again knowingly misleading their customers and the public at large by marketing new Euro 6 diesels as the 'cleanest in history'.
"Their own data shows that more than half of the very newest, top selling, Euro 6 diesel cars pollute far above the latest standards.
"They're still dirty and they're still putting people's health at risk. It's time the industry took responsibility and stopped investing in diesel."
But SMMT chief executive Mike Hawes accused Greenpeace of "deliberately trying to mislead consumers".
He said: "New cars on sale today are the cleanest in history and fully compliant with EU emissions standards."
He said no new models would be allowed to be sold unless they passed the official on-road tests which came into force in September.
Results published so far were part of testing the new methodology and equipment, "but were never intended to be used to demonstrate compliance with a test that was not yet in force", he added.
Air pollution causes an estimated 40,000 premature deaths a year in the UK and is linked to health problems from childhood illnesses to heart disease and even dementia.
The UK has long failed to meet European Union legal air quality limits on nitrogen dioxide, much of which comes from road transport and particularly diesel vehicles.