The Department for Transport has launched a consultation on proposals to strengthen laws governing emissions from vehicles already in use, in a bid to improve air quality and protect public health.

While new vehicles must meet strict emissions standards before they can be sold or registered, ministers say there is currently no legal requirement to ensure emissions control systems continue to perform to the same standard throughout a vehicle's lifetime. As a result, poorly maintained or modified vehicles can emit significantly higher levels of harmful pollutants.

Under the proposals, all road vehicles in Great Britain would be legally required to maintain their emissions control systems so they continue to meet the standards in place when the vehicle was first approved. This would make it illegal to use, modify or allow the use of vehicles whose emissions exceed their original legal limits.

The government says road transport remains the UK's largest source of nitrogen oxides and a major contributor to harmful particulate pollution. Research cited in the consultation found that fewer than one in 10 passenger cars tested met official nitrogen oxide limits, with around two-thirds of Euro 5 and Euro 6 diesel cars emitting more than three times the permitted level.

Ministers argue that stronger enforcement is needed as petrol and diesel vehicles are expected to remain on UK roads for many years despite the transition to electric vehicles. Breaches of the proposed rules could result in fines ranging from £1,000 to unlimited penalties following prosecution.

The consultation is seeking views on the proposals before any legislative changes are introduced.