Some doubts about the safety of a certain part of Guangzhou's northern subway extension that were made public in September have now been laid to rest -- maybe.

It all has to do with claims of substandard concrete work in a passage in the northern extension of Subway Line 3, which were made over the Internet in mid-September.

The whistleblower was Zhong Jizhang, an engineer at the Guangzhou Huijian Center of Engineering Quality and Safety Monitoring, who claimed there had been a cover-up, which triggered worries about the safety of the city's subway projects.

A spokesman for the city's urban-rural construction commission confirmed Zhong's report on Friday, but said that the passage was safe to use and cited a study by a team of experts that the commission had asked to examine it.

The passage in question links two tunnels for work and emergency purposes and is not itself a major tunnel for trains.

The commission's group of experts concluded on Thursday that the concrete issues would have "no decisive impact on the quality of the entire project" and could meet safety requirements.

But the cause of the flaw is still under investigation, the spokesman added.

Zhong, the whistleblower, is a retired engineer who works for the engineering quality center, and who said in his blog that the tunnels had been given the all clear last year in spite of the substandard quality of the work.

The spokesman confirmed that the Guangzhou Metro Corp (GMC) discovered the flaw in October of last year and asked the project's designer to "evaluate the structural safety".

The design institute concluded that, in spite of the concrete flaw, the passage still met all structural safety requirements.

However, Guangzhou's New Express Daily disputed this in a report on Wednesday that said tests by two companies hired by the GMC and the construction company found that the concrete in part of the passage failed to meet design specifications.

When the public began expressing doubts about this last week, the city sent a team of experts to do a thorough check of the passage and look for any flaws in construction work and project monitoring. They concluded that it was safe to use.

The construction commission said they would continue to monitor the quality of the northern extension "in a strict way". The line is scheduled for completion this month so that it will be in operation by November, before the Asian Games.

But, on Thursday, it was reported that Beijing's Chang Cheng Bilfinger Berger Construction Engineering Co Ltd, which handled the extension, has been banned from bidding for any new projects in Guangzhou for six months.

Chang Cheng Bilfinger was established in 1995 as a Sino-German joint venture. The parent companies are Germany's Bilfinger Berger AG and the Beijing Construction Engineering Group.

Guangzhou has plans for its subway network to have eight lines, for a total of 236 km, when the Asian Games open next month. The current six lines handled a record 5.13 million passengers on the national day holiday, Oct 1.