Around 20 celebrities were spied on by British security services and these operations may not have been justified or warranted.
The disclosure came to light at an investigatory powers tribunal brought by campaigning organisation Privacy International.
According to a report by the Guardian, government lawyers on behalf of GCHQ and MI5 showed that between 2009 and 2013, three searches into high-profile people were "not operationally justifiable."
Another 17 searches, carried out by five officers were said to be possibly not "operationally justifiable". The lawyers added that conversations with those officers were not recorded, meaning it is "not possible to ascertain whether they were in fact operationally justifiable".
The report said that these individuals were not notified of the surveillance and officers were warned that if they were caught misusing their powers they could face disciplinary action.
Government lawyers initially refused to release the figures on grounds of "damage to national security", but documents were released on the second day of the hearing following repeated request by Privacy International.
The intelligence officers have since been banned from using the security services' data to "search for and/or access information other than that which is necessary and proportionate for [their] current work".
Millie Graham Wood, legal officer at Privacy International, said the security services should have informed the celebrities involved they were being spied upon. She said it should be a serious concern that there is no procedure to notify victims.
"Without such a mechanism, and in the absence of independent or judicial authorisation, a victim of abuse has no prospect of ever securing a remedy."
