Amnesty International has expressed outrage after a British tribunal confirmed that Britain's government had spied on the human rights organization.

The Investigatory Powers Tribunal wrote to Amnesty on Thursday to notify it that Britain's electronic intercepting agency, GCHQ, had violated its rules when it kept intercepted communications from Amnesty for longer than allowed. The tribunal, however, said the way the communications were obtained was lawful and proportionate.

Amnesty said the tribunal did not tell it when or why the agency was spied on, or what was done with the information obtained.

The tribunal had said in June that GCHQ failed to delete data intercepted from two other rights groups on time. It said Thursday that it was mistaken, and that one of the groups was Amnesty.