MTR's CEO Mr Leong Kwok-Kuen delivered this verdict in the company's latest assessment report of the project, which is part of the Guangzhou - Shenzhen - Hong Kong high-speed line, to the Hong Kong SAR government on July 1.

"There are three reasons behind the overspending," Kwok-Kuen says. "First, the construction period is longer than scheduled; second, it includes unplanned costs such as extra projects caused by geological conditions that we did not expect; third, the construction costs have been increased, including both manpower costs and costs for materials."

The 26km line, which passes entirely through tunnel and will offer speeds of up to 250km/h, was initially expected to open this year. However, this was pushed back to 2017 last year. MTR subsequently confirmed in March that an Independent Board Committee (IBC) was established to study the ERL project and that it was implementing the recommendations made in two IBC reports which would result in a revised estimated cost and opening date.

Mr Cheung Bin-leung, head of Hong Kong's Transport and Housing Bureau, says the additional delay is "unacceptable" and says that the government will conduct a thorough review of MTR's performance to see if someone should be held accountable for the delay and the overspend.

The project achieved a major milestone at the beginning of March with the breakthrough of the Ngau Tam Mei - Tai Kong Po tunnel, which marked the completion of 12.9km of drill-and-blast tunnel excavation required for the project.