Under the proposal by Cheung Kong subsidiary Mutual Luck Investment, 5 per cent of the 80-hectare wetland in Yuen Long will be developed into 19 residential towers of nearly 2,000 flats while the rest will become a wetland reserve.

The approval - with 17 conditions - was given by the Town Planning Board after a meeting yesterday that had been described as the developer's last chance to prove its case.

Mutual Luck had sought and gained four deferrals since the original deadline in August, and the Planning Department, in an unusual move, sought a final one until yesterday.

Most of the conditions require the developer to submit supplementary documents on a wide range of issues including the master layout plan, habitat conservation plan and funding for the wetland reserve.

Conservancy Association campaign manager Peter Lee Siu-man criticised the board for its poor gate-keeping.

"It's like giving a pass to an examination paper then asking you to rework it," he said. "Members still gave it a green light despite so many uncertainties."

Lee said leaving it to government departments to examine whether the developer had met the conditions, instead of asking it for a more satisfactory application, would deprive the public of a chance to monitor the project. He also feared the approval would open the floodgates for other wetland projects, such as Henderson Land Development's much bigger and also long-delayed project at Nam Sang Wai.

WWF Hong Kong, which terminated a partnership with Mutual Luck earlier this year, said the approval was disappointing.

"The decision shouldn't be made without knowing who will manage the wetland and whether they will have sufficient expertise and experience," said Michael Lau Wai-neng, head of the green group's local biodiversity and regional wetlands programme.

The environmental impact assessment of the project was approved in 2009.

Cheung Kong said the approval was encouraging and that the company would follow up on the conditions.