Hong Kong's new electronic auto-toll system may not immediately ease congestion when it's introduced at the city's busiest tunnel, Transport Commissioner Rosanna Law conceded on Thursday.

Drivers will no longer have to stop and pay tolls when using the Cross-Harbour Tunnel from 5am on July 23.

Once the new system launches, the current eight lanes at the toll plazas on either side will be halved.

“The cutting of lanes to jam into the Cross-Harbour Tunnel, especially in the entrance into the Hong Kong-bound tunnel tube, that would be eased,” Law said.

"However, because it's still the most convenient and cheapest tunnel at the moment, the actual improvement of traffic flow would not be obvious...By the time when we have time-varying tolls, coupled with the HKeToll, I think especially during peak hours, for the Cross-Harbour Tunnel, the traffic would be improved."

New charges for using the three harbour crossings will take effect on August 2, but time-varying tolls will be introduced later on.

Law said the HKeToll system, which has already been introduced at the Tsing Sha Control Area, Shing Mun Tunnels and Lion Rock Tunnel, is capable of handling 150,000 vehicles per hour.

It will also be implemented at the Eastern Harbour Crossing and Western Harbour Crossing next month.