Eneko Gogeaskoetxea Arronategui, 44, was taken into custody for his part in some 10 terrorist cases between 1996 and 1997, British authorities said.
They said he is a suspected member of ETA, the Basque separatist group that has killed 829 people since 1968 in a campaign of bombings, shootings, kidnappings and extortion. The group is considered a terrorist group by Spain, the European Union and the United States.
ETA declared what it called a permanent cease-fire in January and has said it is open to letting international observers verify the truce. Spain insists it must lay down arms.
British police said Gogeaskoetxea Arronategui is wanted for allegedly being a member of an armed group, terrorism offenses, possession of weapons, theft and forgery. He was detained following an early morning raid at his home in Cambridge.
Gogeaskoetxea Arronategui was due to appear at London's City of Westminster Magistrates Court later Thursday for an extradition hearing.
Spanish Interior Minister Alfredo Perez Rubalcaba noted that another suspected ETA member was arrested Wednesday in France and said the new detention shows ETA is weak and cannot hide because of good cooperation between Spain and other countries.
"It does not matter where they are. It does not matter where they go. There, there will always be a police officer -- it does not matter what nationality," he said.
Counterterrorism detectives supported by local officers were searching Gogeaskoetxea Arronategui's house and two businesses in Cambridge following the arrest.
Gogeaskoetxea Arronategui was one of several ETA members allegedly behind a foiled bomb plot at the 1997 opening of the Guggenheim Museum in the northern Spanish city of Bilbao. King Juan Carlos was believed to have been the target of the planned attack.
The plot was detected several days before the museum opening. The ETA militants had disguised themselves as gardeners and planted explosives in pots outside the museum. Police uncovered the plan when they asked the two to identify themselves. One officer was died in an ensuing shootout.
One of the gang was arrested shortly after the shootout while Gogeaskoetxea Arronategui's brother, Ibon, who also allegedly took part in the plot, was arrested and jailed in France last year.
A Spanish Interior Ministry statement said police were tipped off to the Cambridge hideout after a Spaniard spotted Gogeaskoetxea Arronategui at a sports club there months ago. It said he had been living in Cambridge with his family for some time.
Rubalcaba said the arrest did not meant ETA had any sort of a cell operating in Britain.
Police in Britain arrested a suspected Basque separatist Thursday who is wanted in connection with a 1997 plot to assassinate the king of Spain.
