Commuters have been getting used to lengthy waits

Already, hundreds of buses, taxis and auto-rickshaws have been ordered off the roads because of their owners' failure to comply.

As a result, much of the city's transport system has been thrown into chaos, with commuters and schoolchildren regularly facing long delays at bus stops.

The final deadline to convert all public transport vehicles to compressed natural gas (CNG) passed last month, after twice being postponed.

Now, all such vehicles must have passed emissions-standards and safety tests to remain on the road.

Court order

India's Supreme Court ordered half the city's 12,000 buses off the roads last month for continuing to produce clouds of thick, black diesel fumes.

Schools had to be closed down for a week to ease the strain.

Even longer delays could be on the way if many vehicles do not pass their pollution tests and are again ordered to stay idle.

Delhi, one of the world's most populous cities, has seen its rates of respiratory diseases spiral upwards, a rise blamed on its heavy clouds of pollution.

But the BBC's Adam Mynott reports from Delhi that some scientific experts question whether the new fuel is the best alternative to diesel.

Badly handled

They argue that, while CNG exhaust fumes may not appear to pollute the air as much as diesel, they actually contain particles that could be even more damaging to public health.

Transport officials in Delhi agree that the changeover has been badly handled.

There are too few CNG filling stations, and often not enough of the new fuel in the city to meet demand.

Some auto-rickshaw and bus drivers have complained of having to wait for as long as six hours to get their vehicles filled up.