Thirteen people wrongly arrested after data investigation mistakes
Innocent people have been caught up in criminal investigations because of snooping blunders.
Thirteen individuals were wrongly arrested while others were visited by police after errors in acquiring communications data.
Mistakes also led to search warrants being executed at addresses unconnected to investigations or welfare checks on vulnerable people being delayed.
The Interception of Communications Commissioner investigated 23 serious communications data errors in 2015. They include mistakes made either by law enforcement agencies or communications service providers, with the majority of errors believed to relate to child abuse inquiries.
Last year, 145 public authorities gathered more than three-quarters of a million items of communications data - which covers the who, when and where but not the content of an email or text message.
The vast majority - 93.7% - of applications were made by police forces and law enforcement bodies, with intelligence agencies accounting for 5.7%.
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.
A large proportion of the errors related to internet protocol addresses - numerical labels assigned to devices on the internet. An IP address is often the only line of inquiry in a child protection case, and it may be difficult for police to corroborate the information before taking action.
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."
Of the 23 serious mistakes, nine were classed as "technical system errors", while 14 were human errors.
As a result: in 1 7 instances search warrants were executed at the addresses of people unconnected to the investigation and/or people unconnected to the investigations were arrested; in six cases people unconnected to the investigations were visited by police; and seven resulted in delayed welfare checks on vulnerable individuals.
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 in a timely fashion.
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."
Four recommendations were issued concerning prisons not having an adequate translation strategy in place to deal with calls or correspondence in a foreign language.
The report said: " This was particularly relevant to those prisons with a high proportion of foreign national prisoners where a small number of inspections revealed that staff were being directed to listen to a large number of calls made in foreign languages but were not being provided with any guidance as to whether the calls should be translated."
A Government spokeswoman said: "Although errors in acquiring communications data occur very infrequently, any error in these circumstances is deeply regrettable - especially when it has significant consequences.
"The Interception of Communications Commissioner made a number of recommendations to improve processes and prevent errors from occurring. All public authorities who received recommendations will be considering very carefully how to put them into practice."
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.
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."