UN human rights experts have told China a new security law for Hong Kong “infringes on certain fundamental rights” and voiced concerns that it could be used to prosecute political activists in the former British colony.

In a rare joint letter made public on Friday, 48 hours after it was sent to the Chinese government, they also said provisions of the new law appear to undermine the independence of Hong Kong’s judges and lawyers, and the right to freedom of expression.

The “open letter” reflected a detailed legal analysis of the national security law imposed in Hong Kong on June 30, which had already drawn UN criticism before its adoption.

The law allows for anything China views as subversive, secessionist, terrorism or collusion with foreign forces to be punished with up to life in prison. Authorities in Beijing and the financial centre have said the law is necessary to ensure Hong Kong’s stability and prosperity.

Critics say the legislation further erodes the wide-ranging freedoms promised to Hong Kong on its return to Chinese rule in 1997 under a “one country, two systems” agreement.

The 14-page letter, posted on the website of the UN human rights office, was sent by Fionnuala Ni Aolain, UN special rapporteur on protecting human rights while countering terrorism.