With International Day Against Racism being marked today, a new EU survey has found high levels of intolerance in Ireland and Britain.
The survey found almost one-fifth of people in this State find the presence of people of another nationality disturbing in their daily lives.
The survey also found almost a third - 30 per cent - of people in the State believe immigrants are more often involved in crime than are non-immigrants.
Greece was the most intolerant state, according to the survey, with 38 per cent disturbed by the presence of other nationalities. Denmark follows with 24 per cent.
The same proportion - 17 per cent - in this State and in the UK found the presence of other nationalities disturbing. The EU average was 15 per cent.
The special "Eurobarometer" survey of 16,000 EU citizens was carried out for the Commission by the European Monitoring Centre on Racism and Xenophobia.
The Taoiseach warned that if racism goes unchallenged it could undermine the very fabric of our society.
Speaking to mark the International Day Against Racism, Mr Ahern said he wished to restate the Government's total opposition to racism and to any manifestation of it.
"Today is a day on which we should reaffirm our determination to realise an Ireland free from racist attitudes, free from bigotry, free from fear or diversity, and eager to embrace cultural diversity. Today is a day on which we publicly and emphatically reject racism and prejudice."
He said our own experience of emigration and of division would suggest that we should have a well developed sense of justice and equality and an aversion to prejudice.
"Today, the International Day Against Racism, provides a valuable opportunity for us to reflect on how racism affects our society and how, in partnership, we can tackle it."
Mr Ahern said that as national preparations continued for the World Conference Against Racism, Racial Discrimination, Xenophobia and Related Intolerance, which takes places in South Africa later this year, a unique opportunity presents itself to challenge racism.
He said a recent positive step was Ireland's ratification of the UN Convention on the Elimination of All Forms of Racial Discrimination.
"Within the year we will present a report to the UN committee to account for how the Government and people of Ireland comply with the terms of this convention.
"It should be the intention of all that, in that report, we can point to real, meaningful progress in eliminating racial discrimination and racism in this country."
The EU survey found only 4 per cent of Spanish people considered the presence of another nationality disturbing.
The UK is facing a decline in "multicultural optimism", the survey suggested, with more than one-fifth - 22 per cent - thinking legally established immigrants from outside the EU should all be sent back to their country of origin. The comparable figure in 1997 was 15 per cent. (Additional reporting by PA)