Spain's prime minister says he made a "clear mistake" by being too optimistic about the prospect of peace talks with the Basque separatist group Eta.
The Madrid airport bombing was Eta's first deadly attack in years
Jose Luis Rodriguez Zapatero said he had predicted an improvement in the peace process - the day before a bomb attack on 30 December claimed by Eta.
He called for a "great democratic consensus against terrorism" on Monday.
On Saturday, more than 200,000 people in Spain protested over the bomb blast that killed two at Madrid airport.
"All Spaniards heard me say on 29 December that I had the conviction that things were better for us than five years ago and that in a year's time things would be better for us," Mr Zapatero told a special session of parliament debating Spain's anti-terror policies.
"Although it is not frequent among public leaders, I want to admit to all Spanish citizens the clear mistake I made," he said.
"Eta wasted the opportunity to contribute... to a better future for everyone, and by this decision Eta strives to prolong criminal activity which has already lasted more than four decades".
Mr Zapatero said Eta had in effect broken the peace process launched in March, and that there could be no dialogue while the group was engaged in violence.
However, he said he hoped Eta would accept sooner rather than later that words were superior to weapons.
He also called for unity among mainstream parties, saying it was the best way to overcome the problem.
The deaths of two Ecuadorean migrants at Madrid's Barajas airport were the first attributed to Eta in more than three years.
The group - which declared a permanent ceasefire in March - is seeking an independent state in northern Spain and south-west France.