HONG KONG (AFP) - China has finally announced the opening ceremony for the world's longest sea bridge, which will connect Hong Kong, Macau and the mainland, but critics hit back on Thursday (Oct 18) over the secrecy surrounding the project.

Construction started in 2009 on the 55km crossing, which includes a snaking road bridge and underwater tunnel, linking Hong Kong's Lantau island to the southern mainland Chinese city of Zhuhai and the gambling enclave of Macau, across the waters of the Pearl River Estuary.

It has been dogged by delays, budget overruns, corruption prosecutions and the deaths of construction workers.

While supporters promote it as an engineering marvel, others see the multibillion-dollar project as a costly white elephant designed to further integrate Hong Kong into the mainland at a time when Beijing is tightening its grip on the semi-autonomous city.

Local media received invites from Beijing's liaison office in Hong Kong on Wednesday to an "opening ceremony" in Zhuhai next Tuesday (Oct 23), with no further details given.

China's President Xi Jinping is reported to be attending the event, but there has been no official confirmation whether the bridge will go into operation that day.

Hong Kong's transport department had no immediate answer on Thursday when AFP asked whether the bridge would be fully commissioned on Tuesday.

Bus companies supposed to be operating on the bridge complained that they were in the dark.

"At such short notice and without any details, how can we make the necessary logistic arrangements?" Mr Eddie Choi, a spokesman for coach operator One Bus Hong Kong Macau, told the South China Morning Post.

The operator's website lists the schedule and prices for the bus rides, but a staff member said the transport bureau had not confirmed the opening date with the firm, and information about ticketing was not yet available.

An official from the mainland-based bridge authority told AFP the bridge would be "considered open" from next Tuesday and confirmed there would be access that day to registered cars and buses, but did not elaborate.

The China Daily newspaper cited a source familiar with the matter saying the bridge would be open to traffic later in the day, after the opening ceremony.

Pro-democracy lawmaker Kwok Ka-ki who sits on the Hong Kong government's transport panel said he learnt of the launch ceremony only from media reports on Wednesday and had not received an invite to the opening.

Members of the transport panel have been invited to a bridge inspection on Saturday, according to Tanya Chan, another panel member.

Mr Kwok accused officials of secrecy and said there were still many unanswered questions.

"The bridge needs to be open and used by the public as soon as possible, but whether it is safe and arrangements are properly in place and conducted and tested, we do not know," he told AFP.

He likened the lack of transparency to the launch last month of Hong Kong's new high-speed rail terminus, which saw Chinese security operating on the city's soil for the first time.

Mainland staff were brought into the station at a hush-hush midnight ceremony.

"Although Hong Kong people have paid a lot for the construction and have a substantial share in this bridge, we have no control," said Ms Chan, who is also a pro-democracy legislator. She added that she had no idea what next Tuesday's opening ceremony entailed.

"The Hong Kong government is always out of the picture and is under the control of the Chinese government," she said.