People who worked on the Grenfell Tower refurbishment could face the threat of being jailed for life, their lawyers have said, with witnesses interviewed by police believing they could be charged with manslaughter.

On Thursday at the public inquiry into the June 2017 disaster, lawyers for current and former employees of the architect, contractor, subcontractor and client on the project said it was plain to those interviewed already by police that the investigation “will include gross negligence manslaughter where applicable”.

“In addition, [detectives] are expressly investigating contraventions of relevant health and safety legislation and regulation,” they said in an application seeking assurances that evidence they give to the inquiry cannot be used to prosecute them.

Health and safety at work crimes are punishable by up to two years in prison, but in the application the lawyers noted that a manslaughter charge could lead to a life sentence.

Jonathan Laidlaw QC, acting for Harley Facades, whose employees are among those already questioned by police under caution and who are due to testify to the public inquiry, said “a significant number” of the witnesses expected to be called in the second phase of the inquiry had already backed the application for protection and more may follow.

“Right through the course of phase two [of the public inquiry] these individuals will remain suspects in respect of whom there is a real and appreciable danger of self-incrimination,” he said.

Many witnesses expected to be called had already been interviewed by detectives, he said, and unless they were given the undertaking then they would invoke their right to refuse to answer even the most basic questions in front of the inquiry.

Without protection it would be “impossible to get to the answers the bereaved, survivors and residents are plainly entitled to,” Laidlaw said.

The tactic was branded “sabotage” by lawyers for the bereaved and survivors, many of whom were furious. They are due to consider their position on the application over the weekend. Sir Martin Moore-Bick, the inquiry chairman, said the witnesses’ move was “very disappointing”, not least because the issue had been raised more than a year ago.

Laidlaw said four witnesses for Harley Facades, the company that oversaw the cladding works, had already been interviewed by police and faced further interviews over possible breaches of the Health and Safety at Work Act.

That act places a duty on every employee while at work to take reasonable care for the health and safety of anyone “who may be affected by his acts or omissions at work”.

“You would need to demonstrate that you had done all that was reasonably practicable to escape conviction,” Laidlaw said. “Any person who has failed to take reasonable care for the safety of another commits a criminal offence potentially punishable by a term of imprisonment.”

Detectives have said they will not consider whether or not to pass a file recommending prosecutions to the Crown Prosecution Service until after the inquiry has concluded. That means any trials could be at least two years away.

Some of the bereaved and survivors have said they would prefer to see justice though the criminal courts than the public inquiry and so would prefer the police investigation to come first.