Catherine Murphy, TD for Kildare North, and Finian McGrath, TD for Dublin North Central, told the Sunday Tribune they initiated the legal action after analysing the final census of population figures, released last month. Those figures showed 11 of the 43 constituencies in clear breach of the constitution's article 16.2.3 that requires every constituency to have, within reason, the same ratio of TD per population.

The TDs' legal team includes senior counsel Gerard Hogan, the top constitutional law expert in the country.

The legal challenge is a major worry for the government which last autumn ruled out changes to the Dail constituencies in advance of the election.

A Sunday Tribune investigation first raised the serious constitutional problem with the constituencies in November 2004 and again warned about a clear breach of the constitution last July, when preliminary census figures were published.

In response to the Sunday Tribune's revelations, the government obtained the advice of the attorney general but, after months of consideration, declared it could not change the constituencies until final census figures were published in 2007 . . . too late to change the constituencies before the election.

There were two weaknesses in the government's argument: the constitution does not differentiate between the preliminary and final figures, and the CSO declared last July that there would be no effective difference between the two figures.

The inaction has resulted in clear inequalities. In next month's election, Dublin North will return just four TDs despite having a higher population than four five-seat constituencies. Dublin West will return just three TDs even though it has a higher population than four-seat Cork-North Central.

Murphy and McGrath said the action was essential to ensure fair elections. "The election is due to be called and it will be outside the parameters of the constitution. That's a fact."