The force used its power to obtain the data in 2013, in an attempt to trace the source of what it believed was an internal leak to the press of the resignation of an assistant chief officer.
The journalists, Thomas Wilkinson and Owen Humphries, were notified four years later, in April 2017, by the force that it had inspected their call records. It also offered an apology.
The admission followed a separate judgment by the same tribunal in January 2017 that the force’s applications to see the call logs of six other people in 2012 were unlawful. This was initially, unsuccessfully, contested by Cleveland’s chief constable.
This judgment prompted the chief constable to commission a review of all Cleveland Police’s applications for authorisations under section 22 of the Regulation of Investigatory Powers Act since 1 January 2011 and undertake to publish its results.
It was this review that brought to light the authorisations regarding Wilkinson and Humphries’ call records, which the force admitted were "neither necessary nor proportionate".
Wilkinson and Humphries subsequently complained to the Investigatory Powers Tribunal, seeking a declaration that the force had acted unlawfully, an order quashing the authorisations, and the destruction of all material and information gained as a result of them.
The chief constable accepted the claimants were entiltled to this relief, and the tribunal found in their favour.
A Cleveland Police spokesperson said: "On 19 February the Investigatory Powers Tribunal (IPT) judged that two Cleveland Police RIPA authorities dating back to 2013 were unlawful.
"The Force had earlier reviewed these authorisations and determined that these were neither necessary nor proportionate. Having done so, apologies were made to those involved and details of what had been found were provided.
"Upon notification that two individuals had registered complaints with the IPT, we conceded that the authorisations were indeed neither necessary nor proportionate.
"In order to understand and learn from these cases, the police and crime commissioner and the then-chief constable commissioned a review of applications by the Professional Standards Department over a six-year period.
"This insight and our Transforming PSD Programme will enable us to continue to improve our approach and ensure that we operate to the highest standards."
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