The RSPCA hid evidence and behaved unlawfully when it put down a family's cat against their wishes, a damning report has found.
Claude, a 18-year-old Turkish Van, was seized from Richard and Samantha Byrnes in 2013 after a neighbour raised concerns over his appearance.
He was taken despite a desperate plea from Samantha to allow her children, then aged 12 and 14, one last chance to say goodbye.
The RSPCA denied her request and the cat was destroyed on the recommendation of a vet due to age, extreme emaciation, matted fur and probable underlying disease.
Samantha argued his appearance was due to his dislike of grooming and had to be put under anaesthesia when being trimmed.
The couple were charged under the Animal Welfare Act 2006 but the charges were later dropped.
The RSPCA issued an apology in 2014 but an unpublished report, which The Times claims to have seen, lays bare the deception from the organisation.
It issued a certificate claiming it had no information which could help the Byrnes' case.
An official review conducted by Stephen Wooler, the former HM Chief Inspector to the Crown Prosecution Service found this not to be the case.
The RSPCA also wrongly claimed two vets had examined Claude and advised its destruction when the Byrnes went public with the case.
The charity then BBC Radio 4 Today programme the cat had specifically been kept alive overnight so the children could say goodbye when in fact this had only been because Samantha had refused to give consent to the euthanasia.
Wooler described this as a "travesty".
He added: "This in turn generated comments, some intemperate and distressing, directed at Mr and Mrs Byrnes and their young family.
"It is necessary to say no more than that it is no part of the role of a prosecuting authority to engage in a war of words through social media."
The RSPCA has powers of prosecution under the Animal Welfare Act 2006.
MPs are to hold a private inquiry into the organisation's role as a private prosecutor.
Wooler said: "The law is quite clear that when sensitive documents contain material that is disclosable, a mechanism must be found of achieving that, or the case has to be discontinued.
"There is no public interest that will defeat the requirement to make disclosure when the interests of justice so require.
"A striking feature of the events is the confidence displayed by [the RSPCA] that the police would unquestioningly acquiesce in whatever actions [it] requested.
"Respect for due process and the rights of individuals was largely absent."
An RSPCA spokesman said: "We have already accepted mistakes were made in our handling of this case. Our goal in commissioning the report was to learn from our mistakes. We have done this.
"Significant improvements and refinements to our processes have been made to help ensure that the RSPCA responds appropriately and proportionately to situations like this which require sensitive handling that takes account of both animal welfare and human issues.
"The RSPCA apologises again to Mr and Mrs Byrnes and their family."