A 2013 report by Dutch research institute TNO showed that diesel car systems in the Netherlands were adapted to appear as if the vehicles were less polluting than they actually were, current affairs programme Nieuwsuur said on Sunday.

The report, which did not name the manufacturers involved, said the nitrogen oxide emissions from ‘a number’ of diesel cars which officially met Euro 6 emission standards were higher than would have been expected in practical situations.

The reports said that systems had been installed so that ‘they operated during functional driving conditions in a test but in real-world situations were (partly) switched off for economic reasons’, Nieuwsuur reported.

The TNO research was carried out on behalf of the transport ministry. A ministry spokesman told Nieuwsuur that the ministry was not aware of the deliberate manipulation of the emissions data. In addition, the ministry had not read into the report the claim that the systems had been adapted or switched off, the spokesman said.

Real world

In a 2015 English-language summary of four reports into diesel car emissions, TNO stated: ‘Almost all Euro 6 vehicles that were measured emitted significantly more NOx in real-world conditions on the road than during a type-approval test in the laboratory.’

The report continued: ‘It is striking that in real-world conditions the NOx emissions are more than eight times higher than the type-approval value.’ TNO is campaigning to have real-world conditions incorporated into official emissions tests, which are currently lab based.

Two German newspapers said on Sunday Volkswagen’s own staff and one of its suppliers warned years ago about software designed to thwart emissions tests.

Last week, Volkswagen acknowledged installing software in diesel engines designed to hide their true emissions of toxic gasses.