The Taoiseach said today his embattled Minister for Transport, Mr Cullen, could face two inquiries over the payment of over €300,000 to a consultant.
Mr Ahern said it was "likely" that two separate investigations - including a fast-track procedure and one by the Standards in Public Office Commission - would be held into the former Minister for the Environment's hiring of the consultant.
"I think it is very likely we will have two. We will have a former civil servant examining it and examining the procedure of the papers and the Standards in Public Office will under their legislation look at the aspects covered by them," Mr Ahern said.
The commission has already ordered a preliminary inquiry.
Mr Ahern said a former civil servant would head the independent inquiry after Labour Party leader Mr Pat Rabitte called for a "speedy" investigation during the week.
The Waterford-based PR consultant, Ms Monica Leech, earned over €300,000 in fees from the Department of the Environment since she was appointed in 2002, according to information supplied to Fine Gael in the Dáil.
Mr Ahern said the civil servant had not been named but they would "get somebody senior."
"We don't want to create any problems for the Standards in Public Office but I think they as on previous occasions they have to go into detail and it will take some time," Mr Ahern told Today FM.
"I think it [the independent inquiry] hopefully won't take too long and there is not that many people involved obviously there is the secretary general, obviously there is the minister."
He added: "I would hope we are talking about a month."