The existence of the tunnel first came to light in November, after lawmakers were found to have been using it to evade supporters of Hong Kong Television Network's failed application for a free-to-air licence, who were demanding Legco support an investigation into the decision.

"This is not a secret tunnel but an emergency passage," Kenneth Chen Wei-on, secretary general of the Secretariat, said yesterday. The passage would remain in place "as there might be emergencies in the complex".

The passage allows lawmakers to leave the Tamar site in Admiralty via the Tim Wa Avenue exit outside government headquarters, rather than the Legco exit, which leads to Tim Mei Avenue, where most protests take place. Access to the tunnel is controlled by a gate.

Chen said he preferred not to impose too many restrictions on the use of the tunnel and hoped that clearly defined guidelines would "lessen lawmakers' guilt" in using it.

But Civic Party lawmaker Dr Kenneth Chan Ka-lok, a member of the Legco Commission, said the existence of the passage would inevitably give the impression that lawmakers were reluctant to face the public.

He urged the Secretariat to establish a strict definition of an "emergency".

Liberal Party lawmaker Felix Chung Kwok-pan, one of those criticised for using the passage, welcomed the idea of clearer rules on the tunnel's use.

But he believes the fact the tunnel is now public knowledge will reduce its use in avoiding protests.

"Everybody knows that we could leave Tamar via the exit of government headquarters now," he said. "It would be no surprise if the protesters surrounded that exit as well next time."

The new government complex at Tamar, built at a cost of HK$5.5 billion, has experienced a series of problems since it opened in 2011, including claims of unfinished fixtures and fittings and the discovery of legionnaires' disease in the water supply.