By Subir Bhaumik
BBC News, Calcutta
Authorities in the Indian city of Calcutta have ordered all vehicles manufactured before 1990 off the roads unless they convert to green fuel.
More than 50,000 vehicles will be affected by the ban
The vehicles have been ordered to switch to liquefied petroleum gas (LPG) or compressed natural gas (CNG) by the end of the year.
Transport department officials say more than 50,000 vehicles will have to be taken off the roads after the ban comes into effect.
Calcutta is India's most polluted city.
"We have given owners of these old vehicles an option," Transport Minister Subhash Chakrabarty said.
"They can take advantage of it if they want to continue using their old vehicles," said Mr Chakrabarty.
A recent study by the Calcutta-based Chittaranjan Cancer Research Institute and Calcutta University indicated that close to 50% of the city's residents suffer from major respiratory disorders.
Cases of lung cancer are also increasing throughout the city because of the high level of air pollution.
Over the past 10 years, several citizen groups have filed public interest law suits to ban old, polluting vehicles from the roads of Calcutta.
Half of Calcutta's residents suffer from respiratory disorders
The state high court then issued directives asking the state government to indicate a definite deadline by when the old vehicles could be phased out.
The government has been asking the court to extend the deadline, saying that the city did not as yet have enough CNG or LPG outlets or garages to covert vehicle engines to green fuel.
Earlier, the authorities banned vehicles manufactured before 1975 from the streets of Calcutta but some old vehicle owners successfully challenged the ban in court.
The court said the state government did not have the authority to ban these vehicles.
But the law has now been amended by the state assembly making it possible for Tuesday's order to be passed.
Transport officials say the ban will affect 9,587 taxis, 7,464 buses, 6,784 auto rickshaws, 1,164 minibuses and nearly 30,000 goods vehicles.