India's environment court Monday ordered all diesel vehicles older than 10 years be deregistered in New Delhi, strengthening a ban on pollution-spewing cars partly blamed for the capital's toxic air, a lawyer said.

The National Green Tribunal directed Delhi's regional transport office to cancel registrations immediately, after police complained of struggling to force the affected cars off the roads.

"Without registration, these (vehicles) will not be able to ply at all. This is effective immediately," lawyer Vardhaman Kaushik, who lodged the petition seeking the order, told AFP.

The tribunal ordered the ban last year on diesel vehicles older than a decade to help bring down Delhi's dangerous smog levels, which the World Health Organization (WHO) ranks as among the worst globally.  

Traffic police say they have issued fines and impounded some 3,000 vehicles, but many reappear on the roads after owners pay the penalty or win court appeals to have their cars released.

Kaushik said Monday's order plugs these legal loopholes and comes after the Delhi and national governments were pressured to clean up the air which chokes the capital for months of the year. 

The Delhi government has announced a string of measures including driving restrictions in January and in April that took around a million cars off the roads for two weeks. 

India's top court has also ordered a "pollution toll" on thousands of diesel-guzzling trucks that enter the capital every night, as well as a ban on large, new diesel cars.

But with nearly 10 million vehicles on Delhi's roads, campaigners say much more needs to be done.

A WHO study of 3,000 cities released in May placed Delhi in 11th position based on annual average concentration of PM2.5 particles.

These particles, less than 2.5 micrometres in diameter, are linked to higher rates of chronic bronchitis, lung cancer and heart disease as they settle deep in the lungs and can pass into the bloodstream.