Bidding for designing the longest sea link, a bridge connecting major cities in the affluent Pearl River Delta, opened in Guangzhou last weekend.

The 29.6-km bridge, to link Hong Kong, Zhuhai and Macao, is estimated to cost up to 72.6 billion yuan (US$10 billion). It will connect the reclaimed island off Gongbei of Macao and the eastern artificial island for the tunnel section of Hong Kong.

At the annual session of the National People's Congress in early March, Premier Wen Jiabao said the bridge construction would begin towards the end of 2009.

The bidding will target the construction design of the main body, ports in Hong Kong, Zhuhai and Macao, and side-linkings in Hong Kong and Zhuhai, the bridge's coordinative office said.

The preliminary design and investigation of the bridge's main body started on March 13, according to a contract signed between the authorities from Hong Kong, Guangdong and Macao.

Authorities reemphasized the protection of the white Chinese dolphins in the Pearl River estuary during the construction.

"Any tendering reports will be required to include efficient measures to protect the sea environment near the estuary," the office said.

The estuary has the largest protection area for the white Chinese dolphins.

Besides, tendering reports should have transport safety-related details as there will be five shipping tunnels under the bridge, it said.