Businessman Mr Denis O'Brien is to tell the Moriarty tribunal he cannot remember a crucial phone call another witness has said was made in December 1995.

Mr O'Brien is due to return to the witness box on Monday, following evidence yesterday and earlier this week that tribunal counsel Mr Jerry Healy SC said has raised issues of credibility concerning Mr O'Brien and witnesses from Norwegian firm Telenor.

Mr Eoin McGonigal SC, for Mr O'Brien, said his client would be telling the tribunal he had no memory of calling Mr Per Simonsen in December 1995 in relation to an invoice linked to a $50,000 (£43,600) donation to Fine Gael.

Mr Simonsen said he was contacted by Mr O'Brien on December 21st or December 22nd and asked to ensure that the late Mr David Austin's name did not appear on an invoice to be issued by Telenor to Esat Digifone.

He was contacted again in January by Mr O'Brien and asked that the invoice, which had been issued, be cancelled and a new one issued for Irish pounds as against US dollars. Mr Simonsen said he was not told why this was necessary.

He agreed with Mr Healy that the change would make it more difficult to connect the invoice issued by Telenor to Digifone, with the $50,000 payment made by Telenor to Mr Austin for forwarding to Fine Gael.

Asked about the matter, Mr Simonsen, who has been involved in start-up operations in a number of countries on behalf of Telenor, said he knew of no other instance where Telenor was involved in a political contribution.

Mr O'Brien told the tribunal earlier this year that the $50,000 was given to Fine Gael by Telenor on its own behalf and that the Norwegian company later "forced" Digifone to reimburse it. He said this happened in May 1996. He also said he knew nothing in early 1996 about the various invoices being issued to Digifone by Telenor.