An explosive document that the government claimed was privileged for 10 years has just been released to the Moriarty tribunal and could potentially exonerate former minister Michael Lowry, Ireland's richest man Denis O'Brien and top civil servants from the Department of Communications.

A legal opinion commissioned by attorney general Dermot Gleeson in 1996 advised Lowry's Department of Communications to proceed with signing over the mobile phone licence to Denis O'Brien's consortium – eliminating a crucial allegation relating to corruption of the political process.

The written advice by senior counsel and Arnotts chairman Richard Law Nesbitt is contained in a document circulated by the tribunal on 2 April. A spokesman for communications minister Eamon Ryan said he had no comment to make when asked why the decision to claim privilege had been reversed.

It is believed that if the tribunal's final report was to conclude the state had acted improperly in awarding the second mobile phone licence, it would create a huge exposure to litigation, potentially costing the taxpayer hundreds of millions of euro by failed bidders.

The department originally claimed privilege on the document which was commissioned when concerns were raised about the late entry into the Esat Digifone consortium by businessman Dermot Desmond, who was then challenging a High Court inspector's report on the controversial Johnston, Mooney & O'Brien site transaction by Telecom Eireann.

The allegation has been made that the department went ahead and awarded the licence to the Esat consortium, knowing it to be in breach of competition rules because of the late change in its makeup.

Nesbitt advised in 1996 that the restructuring of the consortium made no material difference as long as Desmond's share did not exceed 20%. This prompted him to sell 5% to his partners Esat Telecom and Telenor.

Behind the scenes at Dublin Castle, the affected parties and their lawyers are at loggerheads with the tribunal about how the legal opinion should be entered into evidence. The affected parties are demanding that the former secretary general of the Department of Communications, John Loughrey, Martin Brennan and Fintan Towey of the GSM adjudicating committee and, finally, Lowry be recalled to be questioned about the document.

The issue was openly raised on Friday by Denis O'Brien's new legal team at the tribunal, led by his personal solicitor Paul Meagher who secured the record-breaking €750,000 libel damages for him against The Mirror newspaper.

"You told us to engage in correspondence. We have engaged in correspondence," Jim O'Callaghan SC told Judge Michael Moriarty, "but yet a decision hasn't been made in respect of putting that opinion into evidence. And it was confirmed on April 2 that the claim for privilege over it had been waived. And we are just particularly anxious that the importance of that opinion be put into evidence."

Christopher Vaughan, an English solicitor who acted for Lowry in two property transactions, spent all last week in the witness stand and will be re-called to complete his evidence in June. He was represented by three English lawyers, bringing to five the number of legal teams, numbering a total of 17 lawyers, in attendance throughout the week.

The cost of last week's four days of evidence is estimated at nearly €200,000, contributing to an anticipated final bill of up to €100m.