Nearly seven years after construction began, the world's longest sea crossing is set to open.

Key points: The bridge is the sixth-longest to be built in the world

The bridge is the sixth-longest to be built in the world Over 400,000 tonnes of steel has been used to build the structure

Over 400,000 tonnes of steel has been used to build the structure China has plans to use the cities which are linked as a huge economic and business hub

China's engineering feat that links Macau, Hong Kong and the mainland China city of Zhuhai is to begin operating on Wednesday, with Chinese President Xi Jinping to attend the ceremony.

The crossing, built from 400,000 tonnes of steel, spans 55 kilometres across the Pearl River Delta with a 22.9-kilometre bridge over the sea (the world's sixth-longest) and a 6.7km tunnel.

The bridge has also been designed to withstand earthquakes and seasonal typhoons that lash cities around the Pearl River estuary each year.

The longest sea crossing the world has ever seen is ready to open for business. ( Reuters: Bobby Yip )

It also includes two artificial islands and is expected to drastically cut commute times for the combined populations of the three cities — in the vicinity of 9.6 million people — with trips to take 45 minutes, a journey up to three hours shorter than previously possible.

The opening will not be without its dramas though, with several transport operators claiming they are not ready due to a lack of communication from the government.

Matthew Wong Leung-pak, chairman of Kwoon Chung Bus Holdings, is one executive with such a complaint.

"[The announcement] was out of our expectation," Mr Wong told the South China Morning Post.

"The government had said it would give us a two-month notice for preparation but it came all in a sudden."

How the world's longest crossing looks from afar. ( Reuters: Bobby Yip )

The crossing is not only a major feat of engineering but is also part of an ambitious plan for China to create an economic hub in its "Greater Bay Area".

It will play a key role in the transport of goods made on the west side of the Pearl River to the east side, where they will be exported.

The Bay Area, according to Chinese government forecasts published in the South China Morning Post, will have a GDP of $US4.6 trillion ($6.46 trillion) by 2030 when it is hoped that 29,000 vehicles a day will use it.