The party, Bildu, was able to field candidates in Sunday’s election only after a lengthy court battle to prove it was not a mouthpiece for armed separatist group ETA.

Far exceeding expectations, Bildu came second in Sunday’s polls with just over one quarter of the total, behind the Basque Nationalist Party’s 30 percent but well ahead of the Socialists’ 16 percent, results showed.

In the region’s second largest city San Sebastian, it even took the most votes, winning eight seats to seven for the Socialists — who have held power there since 1991 — and six each for the moderate Basque Nationalist Party and Spain’s main conservative opposition Popular Party.

Bildu “has redesigned the electoral map of the Basque Country after the best results ever obtained for the ‘radical left’,” Spain’s leading daily El Pais said.

“The explosion of Bildu,” headlined the centre-right daily El Mundo.

“In addition to the many reasons that the Spanish have to be outraged, there is now another that is extremely worrying,” it said in an editorial, warning that Spain could “face a huge political problem” in the region.

An editorial Spain’s conservative daily ABC said the “radicalisation of Basque politics is the worst news from these elections and could have damaging consequences.”

Spain’s Constitutional Court on May 6 reversed a Supreme Court decision to ban some 250 lists of candidates from the Bildu coalition because of links to ETA’s political arm Batasuna.

By a vote of six to five, judges authorised Bildu, which means “together” in the Basque language, to stand.

Bildu was formed with two legal far-left Basque parties, Eusko Alkartasuna and Alternatiba.

Its creation was almost a last resort, after the Supreme Court banned another newly created Basque pro-independence party called Sortu despite its opposition to violence, arguing it was an extension of Batasuna.

As a result, Bildu became the de facto rallying point for supporters of Batasuna.

Batasuna was banned in 2003 because of its supposed links with ETA, which is held responsible for 829 deaths in a four-decade campaign of bombings and shootings for independence for the Basque Country of northern Spain and southwestern France.

The Spanish authorities say they have greatly weakened ETA’s operational capacity through repeated raids and arrests, and the group has observed a ceasefire declared in January. But ETA has yet officially to renounce its arms.