In a feasibility study released yesterday, the Civil Engineering Department proposed spending $1.5 billion to reclaim 80 hectares of land at the Hei Ling Chau typhoon shelter for the prison.

A further $900 million would be used to build a 2.2km bridge and a 350-metre bypass linking the western tip of the island to Mui Wo and the South Lantau Road. An estimated 700 vehicles a day would use the bridge, although most prison staff would get to the island by boat.

The construction proposal was chosen from eight reclamation options and four road link options studied by consultant Mott Connell. Officials said the proposal had already addressed visual and environmental concerns previously raised over the project.

The new prison is meant to replace eight of the 24 jails now in use.

Under the plan, the complex would house about 7,220 inmates - half the original concept - and they would be held in a series of smaller jails rather than one big building. The sites of the eight correctional institutions to be moved to the new complex would be returned to the government for redevelopment.

But green groups and residents on Lantau insisted that Hei Ling Chau was not a suitable location for the proposed prison. The island is home to the endemic Bogadek's Burrowing Lizard, and there were fears the project would lead to further development on South Lantau.

'The whole thing must be stopped right here before more feasibility studies are done so we can reassess if the island is a good site,' said Clarus Chu Ping-shing, assistant conservation officer of the World Wide Fund for Nature Hong Kong.

Ng Cho-nam, director of the Conservancy Association, said: 'We should not be framed by those construction options. All we need is a comprehensive review on where we should build it.'

Green Lantau Association spokesman Cliff Noffke said neither the prison nor the bridge fitted into the proposed recreational zoning of South Lantau.

'It is absolutely inappropriate to build such a complex in the middle of a proposed conservation area,' he said.

Charles Wong Sze-ping, principal assistant secretary of the Security Bureau, said it had considered alternative sites, like Kong Nga Po near the border, but this was not chosen because the area could be used to boost economic integration with the mainland in future.

Mr Wong said the bureau would also study whether the prison complex could be financed by the private sector, adding that no management services would be contracted out.

Mok Yick-fan, assistant director of the Civil Engineering Department, said: 'We believe we have chosen a site which is in the best interest of the public, but we will continue to listen to their [green groups'] views.'

The public has been given two months to comment on the feasibility study. The government will also seek approval from legislators for a more detailed study.