The Government has decided against a review of the current constituency boundaries despite preliminary census figures which show very wide variations in the ratio of TDs to population across the country.
Yesterday Minister for the Environment Dick Roche said the Government had no option but to rule out a constituency review, following advice from Attorney General Rory Brady that to do so would be unconstitutional.
However, Labour accused the Government of making a "politically-motivated" decision to protect Fianna Fáil seats.
Mr Roche said yesterday that there was "a whole series of case law", along with the 1997 Electoral Act, which found that the constituency boundaries should be based on the final census figures rather than the preliminary numbers.
Preliminary figures for this year's census were produced in early July but the final results are not expected until next March at the earliest, which will not provide sufficient time for a constituency review before the general election.
"We have been advised that provisional census results should not be regarded as having statutory force," Mr Roche said.
"It would be unconstitutional of me or the Government to order a constituency review based on preliminary figures, and it would certainly result in a constitutional challenge."
The figures showed huge variations from the national average of TDs to population across the constituencies, with the most extreme being Dublin West, which is 21 per cent above the average, with Dún Laoghaire more than 10 per cent below it.
He acknowledged that the Constitution states that the ratio of TDs to population should be the same across the country, in so far as practicable, based on the last census.
However, he said that case law, and the 1997 Electoral Act, find that the preliminary figures do not constitute a legal census.
He added that there were significant differences in the last census in 2002 between the preliminary and official results, with one local area being out by 3,700 people.
Asked whether it was appropriate to ignore the preliminary census figures, which show serious Dáil under-representation in some constituencies, he said the Government had no choice.
"Whether I think it's fair or not is not the issue," he said. "I have to make a decision based on the law and the Constitution."
Yesterday Labour environment spokesman Eamon Gilmore called for the Attorney General's legal advice to be published, and he questioned whether a legal definition of the census had ever been made in Irish case law.
"The real danger arising from the Government's decision is that a legal challenge will now be mounted to constituencies that are clearly at odds with constitutional requirements."
Meanwhile, Mr Roche has also launched a €1 million advertising campaign aimed at improving the accuracy of the electoral register.
Council officials are calling to almost every household in the country as part of the process.