Parliament is expected to formally censure Winston Peters tomorrow when it votes on a recommendation by the privileges committee.
The committee decided by a majority of 8-5 that the New Zealand First leader provided misleading information about a $100,000 donation from businessman Owen Glenn.
Prime Minister Helen Clark said before the report was released tonight that it would not lead her to take any action against Mr Peters.
Miss Clark said the process had been politicised, and some MPs made up their minds before hearing any evidence.
Mr Peters has stood down from his portfolios while the Serious Fraud Office investigates other donations to NZ First, but he is still a minister.
He has vigorously denied knowing anything about the donation from Mr Glenn, made in December 2005, and has said he was only told about it in July this year.
But the committee's majority opinion was that he had "some knowledge" of it, and should have made "an honest attempt" to file a declaration.
Because he did not make this attempt, the committee reported that he was in contempt of Parliament's rules.
MPs from the National Party, the Greens, the Maori Party, ACT and United Future supported the majority verdict.
Labour and NZ First MPs submitted a minority report saying they did not agree the evidence was sufficient to conclude Mr Peters had any knowledge of the donation.
The committee's recommendation is a bitter blow to Mr Peters, and he accused those who endorsed it of having decided he was guilty before the hearings began.
"It therefore became a legal charade," he said tonight.
"It had echoes of Zimbabwe."
The committee has ordered Mr Peters to file accurate returns of donations covering 2006, 2007 and 2008 within seven days.
Although the committee's task was to decide whether the donation should have been declared in the MPs' Register of Pecuniary Interests, the hearings had intense political implications because Mr Peters' integrity was at stake.
Mr Glenn, a billionaire who lives in Monaco, came to Parliament to give evidence and said Mr Peters asked him for the donation and thanked him for it after he had sent the cheque.
The money was used to pay the legal fees of Mr Peters' lawyer, Brian Henry, for work on the electoral petition Mr Peters launched after the 2005 election.
Mr Henry told the committee he was the one who had asked for the money, and only told Mr Peters about the donation on July 18 this year because it had become a contentious issue.
Mr Glenn provided telephone records that showed he talked to Mr Peters on December 14, 2005, and minutes after that Mr Henry emailed his bank account details so that payment could be made.
Despite that evidence, Mr Henry and Mr Peters said they never discussed the donation.
The committee's report will be tabled when Parliament sits tomorrow and there will be a debate on it before a vote is taken.
Between them, the parties that endorsed the committee's verdict have sufficient votes for a majority in Parliament.
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