Ninety-seven percent of all modern diesel cars emit more toxic nitrogen oxide (NOx) pollution on the road than the official limit, according to the most comprehensive set of data yet published, with a quarter producing at least six times more than the limit.

The new data, from testing industry leader Emissions Analytics (EA), follows the publication this week by the Department for Transport of emissions results for 37 vehicles, all of which emitted more NOx on the road than the official limit. But the new data covers more than 250 vehicles in more stringently standardised road conditions. EA found that just one of 201 Euro 5 diesels, the EU standard from 2009, did not exceed the limit, while only seven of 62 Euro 6 diesels, the stricter standard since 2014, did so.

Diesel cars must meet an official EU limit for NOx but are only tested in a laboratory under fixed conditions. All vehicles sold pass this regulation but, when taken out on to real roads, almost all emit far more pollution. There is no suggestion that any of the cars tested broke the law on emissions limits or used any cheat devices.

Mayoral candidates in London, the city with the worst air quality in Britain, have seized on the DfT data to call for tighter controls on polluting traffic – including a ban on diesel cars.