Tokyo and its surrounding areas have begun the largest diesel retrofit programme yet seen in the world

Thousands of heavy-duty vehicles, such as buses and trucks, have to be fitted with devices to control diesel particulate matter emissions, or be taken off the road in favour of cleaner replacements.

The exact number of vehicles affected is unclear, but could be as high as 600,000, as all those travelling in the area, wherever they are registered, are covered.

All heavy-duty vehicles registered initially more than seven years ago are affected.

Originally made to conform to 1989/90 emission standards or older, they must now be fitted with approved, 60 per cent efficient particulate matter controls, such as diesel particulate filters.

Vehicles manufactured in line with 1993/4 standards must be fitted with PM control devices of 30 per cent efficiency, such as diesel oxidation catalysts. Passenger cars are not affected.