Lawyers representing those involved in the tribunal's investigations are understood to be examining comments by Michael Moriarty, in his report on Charles Haughey, that his conclusions represented "no more than what should be a reasoned and informed expression of opinion".

Legal teams have already communicated with the tribunal querying the validity of making conclusions in its final report . . . which includes the investigation into the awarding of the second mobile phone licence . . . using that same low threshold of proof.

"There is huge to-ing and fro-ing going on behind the scenes on the legal side, " one source said, predicting: "This will end up in the courts and it will come down to whether or not Moriarty, or Mahon for that matter, has to right to make a judgement on this basis.

"All the big hitters involved now know this is the net issue. It'll be challenged. Look at the people involved, heavy hitters like [Denis] O'Brien."

While a spokesman for O'Brien declined to make any comment, other sources confirmed "everything was being looked at" by O'Brien's lawyers, adding that a number of parties were examining the particular issue of the threshold of proof applied to findings.