Lawyer Ben Jaffey made the comment while telling the investigatory powers tribunal about a secret policy introduced in 2012, The Guardian newspaper reported. The monitoring of the alleged practice was reportedly kept secret as well.
The investigatory powers tribunals' aim is usually to look into the activities of public bodies. Such tribunals are independent of the government.
In June, the UK Intelligence and Security parliamentary committee said that the UK intelligence services had been taking part in mistreatment and rendition of terror suspects in 2001-2010.
Additionally, the committee said in a report that the UK intelligence services were aware of reports of mistreatment in 38 cases in 2002 carried out by the United States.