Independent TD Michael Lowry engaged in a litany of deception - including the falsification of a solicitor's files - in his failure to co-operate with the Moriarty Tribunal, a High Court judge said.
Mr Justice John Hedigan made the comment when he dismissed on all grounds Mr Lowry's challenge to a decision of that tribunal to only award him one-third of his legal costs, a bill he said will run into millions.
The tribunal had found the Tipperary North TD concealed certain dealings, as well as findings of perjury and bribery of a potential witness, to support false evidence, the judge said.
Mr Lowry said his lawyers were already preparing an appeal of the High Court decision to the Court of Appeal.
In a statement, he said: "No matter what way the court decided today, this case is one that was always going to be brought by either side to the highest level for final decision and determination.
"I have never accepted the findings issued by the Moriarty Tribunal as they were not grounded on any evidence or facts presented throughout the length of its sittings."
The tribunal was set up in 1997 to investigate payments to Mr Lowry, a former Fine Gael Communications Minister, and the late former Taoiseach Charles Haughey.
As a result of Mr Lowry's conduct, the tribunal was frustrated and misled and its work was protracted significantly, Mr Justice Hedigan said yesterday.
In light of the tribunal's findings, its rationale for allowing him only one-third of his costs was based on the general ruling criteria of false and misleading information and time lost as a result, he said.
It was also based on the centrality of Mr Lowry in the areas in which he failed to co-operate and the fact that such co-operation could not be reduced to a mathematical formula, he said.
In relation to Mr Lowry's claim that he was discriminated against compared with Mr Haughey, who was awarded all his costs, Mr Justice Hedigan also rejected his arguments.
Mr Justice Hedigan said the tribunal had argued the two men's cases were not the same.
Mr Haughey did not volunteer information in relation to payments to him and "feigned ignorance" of other matters.
"He did not, however, deliberately mislead the tribunal whereas Mr Lowry did," the judge said.