Fine Gael leader Enda Kenny has said there cannot be one law for the Taoiseach and another for everyone else in the country.
Mr Kenny insists that if Mr Ahern had paid his taxes when they were due, he would now be in a position to get a tax clearance certificate from the Revenue Commissioners.
The war of words over the Taoiseach's personal finances continues even with Mr Ahern on an official visit to South Africa.
Last night the Taoiseach's spokesman had insisted in categorical terms that he would not take any questions on domestic politics during the trip.
But in his very first media opportunity Mr Ahern responded readily to weekend claims by the Opposition.
He accused Enda Kenny of telling 'bare-faced lies' when he claimed the Taoiseach had not paid his taxes.
But this afternoon, Mr Kenny travelled to Leinster House from his home in Co Mayo to criticise the Taoiseach at a news conference.
He claimed the Taoiseach's inability to get tax clearance certificate, and his payment of some €70,000 on account to the Commissioners to cover any possible tax liability, proved he had not paid his taxes when they were due.
Labour leader Eamon Gilmore also joined the attacks, saying the Taoiseach failed to realise the seriousness of his tax situation.
In a statement, Mr Gilmore claimed Bertie Ahern risked undermining his contribution to Irish political life by clinging to office when his credibility was so undermined.