Two people have been successfully prosecuted for refusing to provide authorities with their encryption keys, resulting in landmark convictions that may have carried jail sentences of up to five years.
The government said today it does not know their fate.
The power to force people to unscramble their data was granted to authorities in October 2007. Between 1 April, 2008 and 31 March this year the first two convictions were obtained.
The disclosure was made by Sir Christopher Rose, the government's Chief Surveillance Commissioner, in his recent annual report.
The former High Court judge did not provide details of the crimes being investigated in the case of either individual - neither of whom were necessarily suspects - nor of the sentences they received.
The Crown Prosecution Service said it was unable to track down information on the legal milestones without the defendants' names.
Failure to comply with a section 49 notice carries a sentence of up to two years jail plus fines. Failure to comply during a national security investigation carries up to five years jail.
Sir Christopher reported that all of the 15 section 49 notices served over the year - including the two that resulted in convictions - were in "counter terrorism, child indecency and domestic extremism" cases.
The Register has established that the woman served with the first section 49 notice, as part of an animal rights extremism investigation, was not one of those convicted for failing to comply. She was later convicted and jailed on blackmail charges.
Of the 15 individuals served, 11 did not comply with the notices. Of the 11, seven were charged and two convicted. Sir Christopher did not report whether prosecutions failed or are pending against the five charged but not convicted in the period covered by his report.
To obtain a section 49 notice, police forces must first apply to the National Technical Assistance Centre (NTAC). Although its web presence suggests NTAC is part of the Home Office's Office of Security and Counter Terrorism, it is in fact located at the government's secretive Cheltenham code breaking centre, GCHQ.
GCHQ didn't immediately respond to a request for further information on the convictions. The Home Office said NTAC does not know the outcomes of the notices it approves.
NTAC approved a total of 26 applications for a section 49 notice during the period covered by the Chief Surveillance Commissioner's report, which does not say if any applications were refused. The judicial permission necessary to serve the notices was then sought in 17 cases. Judges did not refuse permission in any case.
One police force obtained and served a section 49 notice without NTAC approval while acting on "incorrect information from the Police National Legal Database", according to Sir Christopher. The action was dropped before it reached court. ®
Readers with further information about the convictions can contact the reporter in confidence here.