The National Crime Agency (NCA) has launched an internal inquiry into its use of warrants and production orders following the collapse of major trials, amid warnings that other cases could be in jeopardy.
The NCA and the Crown Prosecution Service (CPS) are now investigating every type of authorisation that the organisation has received to raid homes, seize property, and collect telephone and banking records, BuzzFeed News reported.
The website reported that the NCA admitted to judges it used evidence which may have been gathered unlawfully in four major cases - three of which have collapsed at a cost of millions of pounds to the taxpayer.
The NCA said the review would be a "substantial task" and the organisation, dubbed Britain's FBI, had issued updated guidance to officers and revised training.
Judge Wendy Joseph QC, who presided over three of the trials involved, told BuzzFeed News that the problem was "systemic" in NCA and other cases could be affected.
In a statement, the NCA said: "As a result of issues identified with warrants in two cases that went before the courts, Operations Heterodon and Enderby, the NCA has instigated a comprehensive review of inherited processes and standards around warrant applications, and of live cases where issues with warrants may not have been previously identified.
"The review is of all warrants, production orders, account monitoring orders and authorisations for searches under S18 of PACE (Police and Criminal Evidence Act) in all live cases. It is chaired by a member of the NCA legal team, independent from the part of the NCA which obtained the warrant and is assisted by the CPS.
"This is a substantial task but we are determined to ensure that the issues affecting Operations Heterodon and Enderby are not repeated.
"As well as the review, we have issued updated guidance to officers, revised the legal and awareness training for officers applying for warrants, and introduced additional rigour to the application and authorisation process, ensuring all new applications meet the required standard. We are sharing our experience with law enforcement and police colleagues to ensure that they too can learn from the cases.
"Our cases are complex and challenging, and the nature of the criminals we investigate requires us to be dynamic and assertive in our approach. We are proud of our conviction rate, which was 91% last year, and we do not want any prosecution to fail, particularly if there is anything we can do ourselves to prevent that."
The NCA's deputy director Chris McKeogh said the collapsed trials had been due to "incompetence".
He told BBC Radio 4's Today programme: "It is a matter of record, and in those particular cases a harsh assessment but I think justified, to say that it was incompetence that sat within those two cases."
Pressed on the scale of the internal investigation, Mr McKeogh said: "By the time the review finishes, which I'm expecting to complete in the early part of January next year, we will have looked at in excess of 2,000 different parts of documents that may be relevant to warrants and orders."
He played down the prospect of completed cases being quashed, claiming that there was nothing to suggest that.
"The review started at the beginning of September and there has been nothing found to date that lets me, or indeed the senior panel that is doing this work with me - from the Crown Prosecution Service - to believe that we need to go back to settled cases."
He added: "What we have got is, as we go through the various orders and documentation that we are finding, we are talking about small, technical, procedural flaws."