Such limitations, the expert panel said, do not “begin to meet the standards citizens of Hong Kong would likely require of a police watchdog operating in a society that values freedoms and rights”.
Protests have rocked Hong Kong for more than six months, with up to two million people taking to the streets, initially against a now-shelved extradition bill.
Latterly, one of the core demands of protesters — alongside fully free elections — has been an inquiry into the police, who have been left to battle increasingly violent black-clad activists and are now loathed by significant chunks of the deeply polarised population.
But both chief executive Carrie Lam and the police have repeatedly rejected those calls.
Pro-democracy lawmaker Tanya Chan described the resignation of the expert group as a “vote of no confidence” in the IPCC and the interim report it is expected to produce next year.