Errors in the use of communications data led to the arrests of 13 innocent people, a watchdog has said.
The wrongful arrests were among 23 serious mistakes made in acquiring 761,702 items of communications data, the Interception of Communications Commissioner report found.
Other incidents included delayed welfare checks on vulnerable people.
Communications data includes when and where electronic communications are made, but not their content.
The report found mistakes were made either by law enforcement agencies or communications service providers, with the majority of errors believed to relate to child abuse inquiries.
In these instances often the evidence used was an internet address which was wrongly linked to an individual.
There were six instances in which people unconnected to the investigations were visited by police and seven cases that resulted in delayed welfare checks on vulnerable individuals.
Of the 23 serious mistakes, 14 were human errors and the other nine “technical system errors”.
Commissioner Sir Stanley Burnton’s annual report said: “Any police action taken erroneously in such cases, such as the search of an individual’s house that is unconnected to the investigation or a delayed welfare check on an individual whose life is believed to be at risk, can have a devastating impact on the individuals concerned.”
Overall, 1,199 communications data errors were reported to the watchdog in 2015 – an increase of 20% on the previous year.
Of these, 86.6% were attributable to public authorities, 12.6% to communications service providers and 0.8% to other parties.
Nearly 94% of the requests were by police and law enforcement and just under 6% by the intelligence agencies.
The watchdog also inspected prisons and identified some instances where not all of the calls made by inmates subject to monitoring were being listened to, or that the calls were not being listened to quickly enough.
The report said: “This is of concern because a significant piece of intelligence could be missed completely or not reacted to promptly, leading to a serious incident occurring which may have otherwise been prevented.”
Meanwhile, a separate report, also released on Thursday, revealed that security services made nearly double the number of mistakes using intelligence powers in 2015 than in the previous year.
Almost all of the 83 errors in 2015 led to an intrusion into privacy “to some degree”, the Intelligence Services Commissioner found.
MI5 was also criticised for its form-filling procedures and for inserting unauthorised devices into MI5 systems, such as charging mobile phones, on six occasions.
BBC security correspondent Gordon Corera
Effective oversight is vital for ensuring spies and the police are not abusing their most intrusive powers.
Reports from the Interception of Communications Commissioner provide some of the most detailed evidence, especially when it comes to the increasingly important role of data and technology.
Its annual report – along with that of the Intelligence Services Commissioner also released today – suggests little evidence of deliberate abuse of powers.
But there are still signs of mistakes or sloppy practice.
In the case of communications data, this can be highly consequential with a small number of people arrested in relation to crimes, like suspected child abuse, when the wrong computer had been identified.
In the case of MI5, there also appears to be some problems in the way paperwork is filled out (of which there is far more for police and spies to do than TV dramas suggest) and also in some poor security practice in plugging in phones into secure computers.
There are also still some anomalies in oversight – for instance the use of so-called IMSI catchers to capture phone calls does not come under the Interception Commissioner’s remit.
In a written statement to the Commons, Prime Minister Theresa May said both reports “recognise the diligence and rigour of those who use investigatory powers”.
She said: “These are important powers that are used, when necessary, to keep our country safe.
“Both reports contain details of the recommendations that the commissioners have made to continue to improve the way that these powers are used.
“The public authorities who have received these recommendations will be giving careful consideration to them and how to further improve their processes.”