Haulage firms have been given extra time to adapt their lorries to meet London's new Low Emission Zone (LEZ) anti-pollution rules.
Cars and motorcycles will not affected by the zone
From 4 February, trucks weighing more than 12 tonnes which fail emission tests could be fined up to £200.
Firms will be given up to 28 days after the zone comes into effect to ensure their vehicles meet "green" standards.
Other vehicles, such as buses and coaches, will have to comply with LEZ standards by 7 July.
Reducing traffic pollution
Cars and motorcycles will not be affected by the zone, which covers most of London.
The LEZ will be enforced using fixed and mobile cameras which will read number plates within the zone and check them against a database of registered vehicles which have been adapted to meet the anti-pollution rules.
Failing vehicles will be issued with a warning letter the first time they are seen in the zone.
Nick Fairholme, head of the LEZ, said: "We are making the warning letter and the 28-day period known to operators now, particularly those who have sought to take action in the period up to 4 February."
Transport for London, which is implementing the zone, said LEZ was being introduced to reduce pollution caused by heavy vehicles in the city.
An estimated 1,000 people die prematurely in London every year due to poor air quality, it said.