A key witness in the Moriarty tribunal is expected this week to give evidence that may contradict the evidence of the Taoiseach, Mr Ahern, and a former secretary, Ms Eileen Foy.
The former special adviser to Mr Charles Haughey, Ms Catherine Butler, is reported to be ready to give evidence to the tribunal that, in part, will conflict with the Taoiseach's account of the procedures used for signing cheques from the party leader's allowance.
Ms Butler is scheduled to give her evidence tomorrow. That may be delayed until Wednesday depending on how long it takes Ms Foy to finish giving her evidence.
The Sunday Tribune said yesterday Ms Butler would claim it was general policy in Mr Haughey's office until the end of the 1980s for cheques drawn on the leader's account to be filled in before being given to Mr Ahern for co-signing. Mr Ahern usually signed the cheques before Mr Haughey added his signature.
Mr Ahern was co-signatory of the party leader's account from 1984 to 1992 and his signature was on most of the 1,600 cheques drawn during that period.
However, in his evidence to the tribunal in July the Taoiseach said he used to sign blank cheques drawn on a Fianna Fail bank account and leave them to be filled in or countersigned by Mr Haughey.
He said the practice of pre-signing cheques had been put in place for administrative convenience, due to the volume of transactions through the account combined with the necessity for the regular writing of cheques.
Mr Ahern said a series of cheques would be pre-signed by a signatory on the account, and thereafter the appropriate co-signatory would sign the cheque with the details of the payee and the amount.
He told the tribunal that with the benefit of hindsight his practice of pre-signing blank cheques might have been inappropriate.
Sources said last night that Fianna Fail was likely to have legal representation at the tribunal for the cross-examination of Ms Butler.
A Fianna Fail spokesman said: "That matter has not been considered or discussed yet."