Mr Denis O'Brien has threatened the Moriarty tribunal with a constitutional action.

He is understood to be incensed by the tribunal practice of making opening statements which end with a summary of matters which are to be investigated in public evidence.

He believes this practice creates an impression of being guilty unless it can be proved otherwise.

When the tribunal resumes public hearings in October it is due to begin a huge inquiry into the granting of the State's second mobile phone licence to Esat Digifone in 1995-96.

Mr O'Brien's legal team is understood to have issued a warning about a possible constitutional action regarding the opening statement which will be given before the commencement of public evidence on this issue.

A spokesman for Mr O'Brien said he would not be commenting on the matter.

Last week counsel for Mr O'Brien, Mr Eoin McGonigal SC, expressed his "outrage" at the way he said his client was being treated by the tribunal.

Mr McGonigal said he could not understand why Mr O'Brien was brought back from a holiday abroad to give evidence for 10 minutes on a matter that Mr O'Brien had said he knew nothing about.

The tribunal chairman, Mr Justice Moriarty, said that for the second time in the life of the tribunal, he was noting an expression of outrage by Mr McGonigal.

Outside the tribunal Mr O'Brien said that to describe him as upset would be an understatement.

Relations between Mr O'Brien and the tribunal are understood to be very bad. Last year he refused to give evidence on a particular date because of the birth of his second child. The inquiry into the awarding of the licence to Esat Digifone is expected to concentrate on a number of areas.

These include: the decision not to hold an auction for the licence but rather to set a flat fee; the involvement of Mr Dermot Desmond by way of his company, IIU Ltd; and whether the weightings which were to be used when deciding on a winner were leaked to any party.

The tribunal has discovered five separate transactions which may link Mr O'Brien to Mr Michael Lowry, the government minister who oversaw the licence competition. Mr O'Brien has said Esat Digifone won the competition fairly and that he would welcome a full inquiry into the matter so as to lay to rest the "innuendo" which exists. Mr Lowry has made similar comments.

Ironically, Mr Lowry and Mr O'Brien are said to have become friendly after the announcement that Esat Digifone had won the licence.

There were expressions of discontent from the losing consortiums and Mr Lowry went to the trouble of calling a press conference where the media were invited to ask questions of the civil servants who were involved in the process. Mr O'Brien told the tribunal last year that in his view Mr Lowry had made mistakes but that he should not be disowned because of that.

In 1998 Mr Lowry got involved in UK property transactions with Mr O'Brien's accountant, Mr Aidan Phelan.

This resulted in money which had been given to Mr Phelan by Mr O'Brien for work done, being used in UK property deals involving Mr Lowry.

Mr O'Brien has said he knew nothing of this until it was revealed to him by the tribunal.