Senedd member Neil McEvoy broke rules prohibiting the use of Welsh Parliament resources for election campaigning 22 times, an investigation has found.
A report by the Senedd's standards commissioner said Mr McEvoy "caused or permitted" the printing of thousands of election leaflets in his office.
The politician also allegedly employed a member of staff to translate campaign material into Welsh.
Mr McEvoy denied any breaches and said the investigation was a "witch-hunt".
The standard commissioner's report, yet to be published and leaked to BBC Wales, says the Senedd member committed the breaches ahead of elections in 2016 and 2017.
It lists 22 different examples where Standards Commissioner Douglas Bain alleged that the Member of the Senedd (MS) for South Wales Central had contravened the rules prohibiting the use of resources paid for by the Welsh Parliament for campaigning for elections.
He wrote while "it may be the case" that Mr McEvoy "was not aware of the details of every occasion on which his office was used for non-Assembly work I find it inconceivable that he was not well aware that such use was taking place".
It is the culmination of a long investigation started by former standards commissioner Sir Roderick Evans and triggered by a complaint by Neil McEvoy's former office manager Michael Deem in 2017.
Sir Roderick's involvement ended after his resignation at the end of 2019, after it emerged Neil McEvoy secretly recorded him.
Mr McEvoy had been a member of Plaid Cymru at the time of the allegations and had headed the Plaid group on Cardiff Council - he was expelled from the party in 2018.
Produced last June, the report now falls to the Senedd's standards committee, if it accepts any findings of wrongdoing, to decide if any sanction would be appropriate.
Mr Bain asked the committee to consider if it was a case of an MS making a number of errors, "or whether the extent of the contraventions demonstrates a wilful and persistent disregard" of the rules and guidance.
The report has come to light after Mr McEvoy was excluded from the Senedd without pay for 21 days over his behaviour towards Pontypridd MS Mick Antoniw last December.
According to the report approximately 8,920 "sheets of election campaign material" were produced on a printer located at his regional office in Cardiff.
It apparently included 3,000 leaflets for the 2016 Grangetown Cardiff council by-election and 2,000 leaflets and 1,960 double-sided direct mail letters for the Riverside ward ahead of Cardiff Council elections in 2017. Mr Bain alleged Mr McEvoy caused or permitted the use of the printer, accounting for four breaches of the rules.
Mr Bain also found Mr McEvoy:
- caused or permitted the location of a separate campaign printer and a folding machine at the regional office in 2016 and 2017
- caused or permitted the use of electricity to power equipment used to process party political and election campaign documents
- employed a temporary staff member between October 2016 and April 2017, paid from Senedd funds, to do translation work of a party political and election campaign nature
- used Senedd rooms in December 2016 to interview candidates for the post of campaign group organiser
- caused or permitted a campaign organiser to be based at his regional office and carry out campaign work
Other breaches concerned meetings to discuss party political or election campaign matters in the regional office and the storing of election boards and newspapers in the regional office.
Four meetings of the Plaid Cymru Cardiff Campaign group are also said to have taken place at the regional office, and two took place in rooms on the Welsh Parliament estate itself.
Although Plaid Cymru apparently met the costs of using the printer, the rental costs were paid for from Senedd funds. Mr Bain wrote that the cost to the taxpayer was approximately £89. A "low" estimate of the cost of the overall alleged misconduct was given as £3,450.
In response, Mr McEvoy said the report was "wildly inaccurate" and said the "public will see this leaked report as nothing more than the witch-hunt it is, as the Welsh establishment does all it can to try to stop me defeating the First Minister in May."
"The investigation into me has taken almost half a decade already and the main conclusions are that some meetings took place in my office four years ago, which I informed the Commissioner about on day one, in addition to about £70 of printing and some leaflets being in my office, which I deny," he said.
He said the "only accurate" aspect to the report was a comment by Mr Bain that the complainant was not regarded as an "entirely reliable witness" and that revenge "was amongst Mr Deem's motives". The complaint was made after Mr Deem was suspended by Mr McEvoy in July 2017, before he was dismissed in February 2018.
Mr McEvoy said politicians from Labour and Plaid were recently found in breach of using the Welsh Parliament estate for party political meetings.
The cases, published in standards committee reports in 2020, involved a Labour Party group meeting where the 2019 general election was discussed, and a 2017 Plaid Cymru general election meeting with MPs and MSs. "The media didn't report this and no further action was taken," he said.
After BBC Wales published this report, Abolish the Assembly Party MS Mark Reckless wrote to the standards commissioner saying he was concerned about the leak and asked him to find the source.
Mr Deem said he could not comment on the contents of a confidential report, but he accepted his involvement in the breaches "was wrong".
"It's about time Neil held his hands-up and admitted what he did, rather than resort to these relentless Trump-like conspiracies," he added.