The state government from November 1 will prevent the entry of inter-state buses into the city from 8am to 6pm in an attempt to decongest the central business district and reduce the carbon content in the city's air during the day.

The inter-state buses - from Odisha, Bihar and Jharkhand - mostly enter the city in the morning and remain parked at Esplanade or Babughat.

"Once the new system takes effect on November 1, the buses will be allowed to enter till 8am, that is before the office-bound traffic hits the roads," a transport department official said on Monday. "And they will not be allowed to start their return journey till 6pm."

Sources in the transport department said the curbs would later be extended to intra-state buses.

"The state government will waive the toll fee on Vidyasagar Setu for long-distance buses that will terminate at the Santragachi bus depot," transport secretary Alapan Bandyopadhayay said. "This (encouraging long-distance buses to terminate at or start from Santragachhi, instead of Babughat or Esplanade) is a step towards freeing Calcutta's lungs of pollution and make the riverfront look cleaner."

The decision follows a Supreme Court order in September 2011, upholding a Calcutta High Court ruling barring morning-walkers from parking their vehicles around Victoria Memorial and ordering the state government to shift the Esplanade bus terminus to a spot at least 3km from the marble monument.

The high court had issued the ruling on a public interest litigation filed by environmentalist Subhas Dutta in October 2002, pleading for steps to protect Victoria Memorial from air pollution.

The court had appointed an expert committee to suggest how the monument could be saved. The panel - comprising officials from the central and state pollution control boards, and experts from National Environment Engineering Research Institute and other organisations - suggested several measures, including a ban on vehicle parking near Victoria Memorial.

"The restriction on the movement of long-distance buses doesn't make much sense. Most of these buses operate between evening and early morning and the new order doesn't address the core issue," Dutta told Metro.

Senior officers of the Calcutta traffic police feel the move would help reduce traffic congestion around BBD Bag during the rush hours. The traffic police were represented in the meeting the transport department held during the day with bus operators from Bihar, Jharkhand and Odisha.

Officers recounted how a similar curb on lorries had made more space on the city roads.

"But the new order will allow long-distance buses to leave the city from 6pm. That could pose some challenge to managing the evening rush- hour traffic," an officer of the traffic police said.