Leave.EU funder Arron Banks has pulled out of a scheduled grilling by MPs investigating "fake news".
The prominent Brexit campaigner was due to appear alongside colleague Andy Wigmore before the House of Commons Digital, Culture, Media and Sport Committee on June 12 to answer questions about the use of targeted online advertising.
But he accused the committee of "conducting a co-ordinated witch-hunt of Leave groups" and said he and Mr Wigmore were no longer willing to appear at the hearing.
In a letter to chairman Damian Collins, Mr Banks alleged that the committee provided support to a pressure group pursuing legal action against Leave.EU in the US courts by providing it with early access to evidence from a witness in its inquiry.
He said he would be reporting the committee to the House authorities for collusion with the Fair Vote Project.
The move comes a day after the House of Commons voted to require the director of rival Brexit group Vote Leave, Dominic Cummings, to appear before the committee, after he rejected a summons from Mr Collins. Failure to do so could result in him being found in contempt of Parliament.
Facebook founder Mark Zuckerberg has also resisted requests to give evidence to the inquiry, despite being warned that he faces a summons if he ever visits the UK.
The Fair Vote Project claims that data was sent to the University of Mississippi and used in the Leave.EU referendum campaign. It has launched legal action in the US state and is seeking an injunction to prevent the university from destroying any data it holds.
Mr Banks said Fair Vote activists used testimony from witnesses to the committee's inquiry as evidence in the court case, which is due to conclude on June 12.
And the millionaire businessman claimed that the committee "released evidence early from a witness, a few days before the hearing in order to aid this group".
He said: "We believe the actions of the committee amount to collusion with a pro-EU campaign group in order to create 'fake news'.
"I am sure you will appreciate the irony of the situation that a committee that is meant to be looking at the role of fake news on politics, is actively supporting the actions of such a partisan group.
"We will be reporting yourself and the people involved to the appropriate House of Common authority for colluding with the Fair Vote Project and investigating whether the Parliamentary privilege you enjoy might be revoked as a result.
"It is perfectly clear that the committee, which comprises only of Remain-supporting MPs, is conducting a co-ordinated 'witch hunt' of Leave groups, involving the Electoral Commission and the ICO (Information Commissioner's Office). You have called no witnesses from the Remain campaign or associated groups."
Mr Banks also said he was lodging a legal appeal on June 12 against Electoral Commission findings that Leave.EU broke spending rules during the referendum campaign.