world's longest bridge china
When completed by 2016, officials say it will be the world's "longest sea-crossing bridge" - spanning nearly 50km (30 miles).
One branch of the bridge will reach Zhuhai in Guangdong province.
The bridge will substantially cut driving times as China tries to make the region a global economic centre.
"Through a more convenient and fast transport network, Hong Kong's financial, tourism, trade and logistics and professional services can become better integrated with the Pearl River Delta and the surrounding areas," said Donald Tsang, Hong Kong's Chief Executive.
'Earthquake-proof'
Low-cost manufacturing is concentrated in Guangdong, but the bridge will make the less developed western parts close to the gaming centre of Macau more accessible.
The bridge will be a six-lane expressway that can handle earthquakes up to magnitude 8.0, strong typhoons and the impact of a 300,000 tonne vessel, said Zhu Yongling, one of the officials leading the project.
Thirty-five kilometres of it will be over water.
Driving times between Zhuhai and Macau to Hong Kong will be cut from three hours to about 30 minutes.
The estimated cost will be $10.7bn (£6.5bn), to be shared between Hong Kong, Macau and mainland authorities.
Environmentalists have objected that the bridge will affect the marine ecosystem of the Pearl River Delta and further endanger the rare white dolphin.
Mr Zhu said the environment and marine life would be protected during construction.
Hong Kong and Macau were returned to China as autonomous regions after being ruled as colonies by Britain and Portugal.
Read more: bbc
