The director of the Competition Authority, Bill Prafiska, has been called on to explain why the merger of Topaz Energy and Statoil is set to go ahead without the authority making a determination on the proposal.
The deadline for a decision passed on Monday without the Authority issuing a ruling or requesting further information because of a mix-up in dates.
Topaz, which last year bought out Shell's retail business in Ireland, is set to become the largest fuel distributor in the State if the deal goes through as expected by the end of the month.
Labour Party spokesman on natural resources Tommy Broughan said the "cock up" at the Competition Authority meant it was now too late act on any reservations it had about the proposal.
"At a time when high fuel prices are making life a misery for so many people it's crucial that there is real competition in the sector.
"If the Competition Authority has any concerns over this deal we'll never know. The director of the Authority must be accountable for this," Mr Broughan said.
Fine Gael enterprise spokesman Paul Coghlan said the error meant "the competition watchdog failed to protect the public interest."
"It is important that the Authority be held accountable for the error. The cock-up needs to be investigated urgently to ensure that the Authority does not make a similar error again," Mr Coughlan said.
Minister for Enterprise Micheál Martin should appoint a senior official to investigate the error and the Authority should appear before the Oireachtas Committee on Enterprise and Small Business, he added.
A spokesman for the Authority declined to comment on the criticism but said Topaz had voluntarily agreed to discuss certain matters with the Authority, which would address some of the issues.
Topaz is a vehicle for Ion, a venture capital consortium backed by telecoms and broadcasting entrepreneur, Denis O'Brien. It is set to pay around €280 million for Topaz giving it direct control of more than 100 service stations and with distribution agreements with another 300 dealer-owned stations.
Chief executive of Topaz Danny Murray, said this evening: "This clears the way for the creation of a major Irish-owned fuels distribution business for the first time in our modern economic history.
"Topaz will be a major force for competition in this sector. We are an Irish company entirely dependent on our ability to be successful in the Irish market and that's got to be good news for Irish consumers, Irish businesses and the economy generally."
Statoil's fuel distribution business in Ireland includes oil importation facilities in Dublin, Cork and Galway with 12 regional depots and 69 service stations.
The company also sells gasoil, diesel and kerosene into all sectors of the market both directly and through a countrywide network of authorised distributors and joint venture partners.
Under the terms of the deal Topaz will also acquire worldwide rights to the "Fareplay" brand which it says is the premier forecourt retailing brand in Ireland.
Mr Prafiska, who was formerly the aviation regulator, was appointed to the Competition Authority at the start of the year.