Unions representing Aer Rianta workers have accused the Minister for Transport, Mr Brennan, of being "hell-bent on misusing Exchequer funds" in the break-up of Aer Rianta.
Mr Arthur Hall of the Technical, Engineering and Electrical Union (TEEU) claimed the Minister was deliberately ignoring the advice of consultants and Cabinet colleagues in moving towards the break-up of the semi-state company.
Commenting on recent leaks from consultants' reports suggesting some Aer Rianta airports will be loss-making, and that the Minister for Finance is concerned about aspects of the proposed break-up, Mr Hall said the proposed break-up was "a waste of taxpayers' money".
The report by PricewaterhouseCoopers - leaked last week - predicted Aer Rianta's debt could rise to €400 million after the break-up.
Mr Hall was speaking following a meeting between Aer Rianta unions and Labour Party TDs - including Labour leader, Mr Pat Rabbitte - to discuss the Government's proposals.
The Labour Party's spokeswoman on transport, Ms Róisín Shortall, who was at the meeting, called on the Government to await the advice of an independent consultant's report it had commissioned before proceeding with the break-up.
The report, which will examine all aspects of the preparations and procedures for the break-up, was recently commissioned by the Government, but is unlikely to be completed until the middle of next year.
Yesterday, the Tánaiste, Ms Harney, announced that legislation to break up Aer Rianta into different airport authorities would be brought before the Cabinet by Christmas.
However, Ms Shortall described the break-up as "an economic and legal quagmire", claiming it would be "exceptionally difficult to legislate for the break-up of Aer Rianta plc".
Last night, a spokesman for Mr Brennan rejected the concerns of Labour and the trade unions as "absolutely unfounded".
The break-up was going ahead, and had the full support of business and local interests in the Shannon and Cork areas, he said. "This will give a debt-free start to Cork and Shannon, and it is agreed Government policy," he said.
Meanwhile, Garda authorities are expected to be asked to investigate the leaking of a confidential Cabinet memorandum on the break-up, details of which Mr Rabbitte revealed in the Dáil on Tuesday. He said it detailed concerns from the Minister for Finance, Mr McCreevy, that the proposed break-up was premature.
Yesterday, Ms Julie O'Neill, secretary general of the Department of Transport, began an investigation into the leak.
A spokesman for the Department said Ms O'Neill had already been in contact with Garda authorities. A decision on whether to refer the matter for criminal investigation would be made at the end of the inquiry.However, Government sources said a criminal investigation into the leak was "almost certain".
Meanwhile, Mr Rabbitte withdrew comments he made in the Dáil in which he asked Mr Brennan what he had received from Ryanair and Michael O'Leary.However, he believed proposals for a second terminal at Dublin Airport were dictating the events surrounding the break-up of Aer Rianta.