The intelligence agencies MI6 and GCHQ may have illegally authorised informants to commit serious crimes in the UK, a security tribunal has heard.

The revelation came as a summary was released of a previously closed judgment by the investigatory powers tribunal, which oversees complaints against government surveillance and spy operations.

The ruling said the agencies’ informants, known as covert human intelligence sources (CHIS), may have been allowed “to participate in criminal activity in the United Kingdom”, and raised the question of “whether that conduct was lawful”.

MI6 appears to be operating the policy despite parliament having given the spy agency powers to break the law only overseas, under section 7 of the Intelligence Services Act, the tribunal heard.