It was the terrorist group’s third attack this year on the proposed Olympic installations in the San Blas district.

The authorities were expecting further attacks but Eta had no difficulty parking a stolen car beside a gymnasium in an area supposed to be monitored by a private security company.

The blast came 12 days before the International Olympic Committee (IOC) is due to announce the host city for the 2012 Olympics. Eta telephoned a warning about the bomb at 7pm on Saturday, allowing police enough time to evacuate 150 building workers and several residents.

A thick plume of smoke signalled that the bomb had gone off. No one was hurt but several nearby cars were wrecked. The stadium and adjacent buildings received only superficial damage.

Alberto Gallardón, the Mayor of Madrid, said: “I am sure that the members of the IOC will not be affected by this Eta bomb that attempts to prejudice our candidature.”

The bomb — up to 20kg of chemical explosive — was in a Renault 19. Because the car had been stolen in the Basque capital, Vitoria, on May 29, police believe that the Eta unit responsible was not based in Madrid.

A government security spokesman said that Eta had been expected to continue its campaign against tourism and Spanish economic interests and that “Madrid 2012 met both criteria”.

In April, French police detained an alleged Eta leader, Peio Ezkizabal, who was in possession of plans of the Madrid stadium and other Olympic installations. He had obtained them from the official website.

José Antonio Alonso, Spain’s Interior Minister, described the weekend attack as another example of senseless violence. He added: “The security forces will continue to work with firmness until they obtain the definite end of Eta.” The bombing coincided with the swearing in, for a third term, of Juan José Ibarretxe as Basque regional leader. His Basque Nationalist Party failed to get a majority in recent elections but he received the necessary votes from a Communist party that supports Eta.

The stadium attack was the sixth by Eta since the socialist Spanish Government gained approval in Congress to start dialogue with the organisation.

On June 19 Eta announced that it would not attack elected politicians but it now seems clear the group has no intention of disarming prior to peace talks, a stipulation made by José Luis Rodríguez Zapatero, the Prime Minister.

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