China today announced it had begun construction of the world's longest sea bridge – barely 18 months after opening the current record-holder.

The Y-shaped link between Hong Kong, Macau and China will be around 50km (31 miles) long in total, 35km of which will span the sea, said the state news agency Xinhua. Due to be completed by 2015, the 73bn yuan (£6.75bn) cost of the bridge will be shared by the authorities in the three territories.

The structure also includes a 5.5km underwater tunnel with artificial islands to join it to bridges on each side. According to the engineering group Arup – which has helped with the design – it is the first major marine bridge-and-tunnel project in China. But the engineering firm described the structure as 38km in length; the reason for the disparity was unclear.

Work is expected to begin with land reclamation to create an artificial island of around 216 hectares (540 acres) off Zhuhai. This will become the customs point for those making the crossing.

But much of the structure will be prefabricated offsite, so, for example, the concrete deck sections can be produced at the same time as the foundations are laid. The tunnel will be made of precast sections – each 100 metres long.

"It is designed with a service life of 120 years. It can withstand the impact of a strong wind with a speed of 51 metres a second, or equal to a maximum Beaufort scale 16 (184 to 201km/h)," said Zhu Yongling, an official in charge of the project construction. "It can also resist the impact of a magnitude-8 earthquake and a 300,000-tonne vessel."

A computer-generated image of the £6.75bn bridge, much of which will be fabricated offsite

Six lanes of traffic will pass across the bridge at a maximum speed of 100kmph, cutting driving time from Hong Kong to Zhuhai from four hours to one.

The bridge was first proposed in 1983 as a way of fostering economic ties between China, Hong Kong and Macau. But it will be particularly welcome as the Pearl River Delta – for many years the hub of China's manufacturing – is buffeted by economic problems. The area's attempt to move up the value chain, combined with the rise of the yuan and the global economic crisis, has seen exports plummeting.

The bridge is one component in a plan issued in January by China's top economic planning body, the National Development and Reform Commission, which aims to fuse the area and the two special administrative regions, Hong Kong and Macau, into one of the world's most vibrant economic centres by 2020. In particular, the government hopes it will help to develop the western side of Guangdong province.

"It is a move for Hong Kong, Macau and the Pearl River Delta region to cope with global economic downturn, boost investment and inspire people," said the vice-premier, Li Keqiang, at the inauguration ceremony in Guangdong. "Meanwhile, it can also further increase [their] links and promote economic co-operation."

Hong Kong has said the bridge should generate $HK45bn (£3.6bn) of economic benefits within the first two decades of use, Reuters reported.

According to an article in New Civil Engineer magazine earlier this year, the bridges cross three navigation channels while the tunnel goes under a fourth.

"There is an airport nearby, so we could not build a bridge [in that area] which was the reason for the tunnel. The immersed tube is the longest in the world at 5.5km long," Naeem Hussain, global bridge leader at Arup, told the publication.

He said the bridge's piers would each be 170 metres high and that the design team had minimised the structures impact on estuary flows by limiting the size and number of columns in the water.

But the WWF and other environmental campaigners have warned that construction could devastate marine ecosystems and endanger the rare Chinese white dolphin, which is found in the estuarine waters of the Pearl river. Officials say they have already considered environmental issues in planning the project.

"We will control the construction noises and turbidity of seawater, and prevent oil pollution," Zhu told Xinhua.

It is only a year and a half since China opened a 36km span across Hangzhou Bay – in the eastern province of Zhejiang – which is currently the longest sea-bridge.

Wang Yong, the head of that project, said the design had led to more than 250 technological innovations and engineering breakthroughs, many of which will no doubt prove useful in building the new construction. He added that the Hangzhou bridge survived 19 severe challenges, including typhoons, tides, and geological problems during the three and a half years of construction.

The longest water-spanning bridge in the world is the Lake Pontchartrain causeway bridge in New Orleans, at 38.4km. But officials said that Hangzhou was a particularly difficult site to build because of its complex climate.