Reform UK's leader and deputy leader, Nigel Farage and Richard Tice, are suing the National Crime Agency (NCA) over the alleged leaking of financial information.
In an interview on Talk TV, Tice accused the agency of leaking "individual bank statements, bank transfers, details of conversations with bank managers from different banks".
He said, the BBC reports, both he and Farage would be seeking an apology and damages for what he called "multiple criminal offences".
It comes after Tice wrote to the NCA last month asking it to investigate whether it was responsible for private financial information being leaked to the media.
That followed reports that the mother of convicted fraudster George Cottrell had made payments to Tice's company Britain Means Business, something that was flagged to the NCA in its suspicious activity reports (SARS).
SARS alerts law enforcement to cases of potential money laundering, and are not in themselves crime reports.
A Reform spokesperson confirmed claims had been filed at the High Court.
A spokesperson for the NCA told the BBC: "Following complaints brought by Richard Tice MP and Nigel Farage, the NCA's Professional Standards Unit is conducting an investigation. That work is ongoing. It would not be appropriate to comment further at this time."