Mikel Garikoitz Aspiazu Rubina, known by his alias Txeroki, was being held with a woman at Cauterets ski resort near Lourdes in the French Pyrenees in a coordinated operation between French and Spanish security forces.

French President Nicolas Sarkozy hailed the arrest as demonstrating "the excellent collaboration between France and Spain in the fight against Basque terrorism."

Spanish political parties also welcomed the arrest as a big blow to ETA.

Txeroki, 35, has been the reputed head of ETA military operations for the past five years. He has been held responsible for ordering and planning several bomb attacks, including one which killed two Ecuadorian immigrants at a Madrid airport underground parking lot in December 2006.

That attack prompted Prime Minister Jose Luis Rodriguez Zapatero's government to end a secretive, six-month attempt at peace talks with ETA.

The woman detained along with Txeroki was thought to be Leire Lopez Zurutuza, who was also suspected of participating in several attacks. Police searched the apartment, which had reportedly been rented a week earlier, in the presence of the suspects.

Separatist beginnings

Born in the Basque city of Bilbao, Txeroki first joined radical separatist youths involved in acts of street violence. He was believed to have joined ETA in 2000, participating in several attacks including one that killed a judge in 2001.

Txeroki was also suspected of personally shooting dead two Spanish policemen in Capbreton, southwestern France, in December 2007.

The two plain-clothed policemen, Raul Centeno, 24, and Fernando Trapero, 23, had been taking part in a surveillance operation with French police when they were shot outside a cafeteria.

Txeroki was believed to have been coordinating ETA cells since 2004 and was Spain's most-wanted man.

Lopez Pena arrest

Txeroki's arrest followed that of five other ETA operatives, including top-level leader Francisco Javier Lopez Pena, in France in May.

Last week, French police detained two suspected members of ETA during a routine police check as they were cycling near Tarascon-sur-Ariege in the south of France.

Less than a day after Txeroki's arrest, another high-profile ETA separatist, Juan Ignacio de Juana Chaos, turned himself in at a Northern Ireland courthouse.

De Juana Chaos walked into the Laganside Courthouse a week after a warrant was issued for his arrest.

He was released from jail only in August after serving 20 years for killing 25 people in 11 attacks.

ETA, which has killed more than 800 people in its campaign for a sovereign Basque state carved out of northern Spain and southern France, has long had a part of its infrastructure based in France.

ETA has been listed as a terrorist organization by the European Union and the US.