Delivery vans used by Amazon, Royal Mail, supermarket chains and others have been revealed to be one of the biggest sources of air pollutants.
Their engines emit up to 23 times the level of toxins permitted under UK law, according to tests published by the Sunday Times.
On-road emissions tests carried out on 26 vans found that the Mercedes-Benz Citan 1.5-litre diesel was one of the worst polluters – emitting more than 20 times the legal limit of 0.08g.
However, the van was well under the threshold in laboratory testing and passed all the official requirements. A Mercedes spokesman said "there can always be differences from the legally prescribed standard in the laboratory during on-road measurements.”
Some have put this discrepancy down to the fact official tests are carried out on empty vehicles with just a driver on board.
Nick Molden of Emissions Analytics, the firm which carried out the tests, told the Times: “Clean-running vans are essential if we are going to clear up urban air.
“Deliveries to homes and businesses are surging and these vehicles do high mileages, often in urban areas, carrying big payloads.”
Mike Hawes, chief executive of the Society of Motor Manufacturers and Traders, said: “It is difficult to comment or compare results from unofficial tests where the methodology is unclear, as real-world emissions always vary depending on load, road and vehicle conditions.”
It comes amid a purported toxic air “crisis” on London streets. Mayor Sadiq Khan yesterday said cars should be banned from roads near schools in order to reduce air pollution.
He accused the Government of “ignoring” toxic air and criticised Philip Hammond for not raising taxes for the most polluting vehicles in Wednesday’s Budget.
A recent study found that tens of thousands of children in London's schools are exposed to illegal levels of air pollution that can damage their health permanently.
The report also shows that London's poor are far more likely to be living in areas affected by air pollution linked to 9,000 early deaths every year in the capital.