Chinese police will gain new legal powers to detain suspects for up to six months without telling their families where or why they are held, according to a state newspaper's account of planned reforms.

Human rights activists and legal scholars warned that the change would legitimise an alarming pattern of detentions under the residential surveillance law, which was initially intended as a less punitive measure than formal detention.

Most of those who went missing in a crackdown on activists, dissidents and lawyers this year were taken to secret locations chosen by police. They were held for weeks or even months under residential surveillance. The law does not specify that relatives must be informed, presumably because it was assumed suspects would be held at their homes. In comparison, police must inform relatives within 24 hours of detention and must seek prosecutors' approval for arrest within 30 days.

The proposed changes are part of an overhaul of criminal procedure law now being considered by the National People's Congress (NPC), China's legislature.

"The proposed 'reform' is designed to legitimise this blatantly unfair, police state practice, while leaving the rest of the criminal procedure law as misleading decoration," said Professor Jerome Cohen, an expert on China at New York University's School of Law.

He said he believed police were specifically banned from holding a suspect at an address other than his or her home under binding rules issued in 1996.

Cohen added: "This is a perfect illustration of the dangers of revising the law in repressive times. The problem is that the police use each law revision round to legitimise their convenient practices and they ignore in practice the legislative and administrative reforms designed to bind them."

The draft text has not been published, but the Legal Daily newspaper reported that police would be able to hold suspects in state security, terrorism or major corruption cases at a "designated residence", if holding them at home would impede the investigation. The decision would need to be approved by higher officials.

In state security and terrorism cases the police would not have to notify the suspect's family if they believed it might hinder the investigation – a criterion that scholars say is so vague as to be meaningless. Experts say residential surveillance has been misused in the past.

But Dr Flora Sapio, of Turin's Centre of Advanced Studies on Contemporary China, said its use against dissidents was a new phenomenon.

Sapio, who has been logging proposed amendments, warned that if the change went ahead it would "entrench the powers of police much more formally" and make it harder to criticise disappearances.

Joshua Rosenzweig, an independent human rights researcher in Hong Kong, cautioned that the changes were still at draft stage and might contain restrictions on the use of the measure, but said the change appeared to be "a dangerous legitimisation of a highly suspect practice".

Scholars have praised other amendments for improving protections for individuals, at least on paper. Changes include ruling out the use of confessions obtained through torture and granting the right to judicial review for mental health patients who are forcibly detained.

Nicholas Bequelin, of Human Rights Watch, added: "If these are indeed the [residential surveillance] provisions, and they are adopted, it seems to be a trade-off for the public security bureau to sign off on the improvements in criminal procedure law.

"There is in any case very little restraint on what the police can do in sensitive cases; what's worrying is that this would be a tacit endorsement … It is a very dangerous precedent."

No one at the Ministry of Justice or the NPC was available for comment.