A man has been arrested by British police for alleged involvement in the Madrid bombings that killed 191 people in March 2004.
The Madrid bombing killed 191 people in March 2004
The Metropolitan Police said Moutaz Almallah Dabas, 39, a Spanish national, was arrested in Slough, Berkshire, on Saturday morning.
He was arrested on an extradition warrant from the Spanish authorities.
He is now thought to be in a central London police station and will appear at Bow St Magistrates' Court on Monday.
A Scotland Yard spokeswoman said: "Mr Dabas was arrested under a European arrest warrant issued by the Spanish authorities for alleged terrorist offences - in particular involvement in the Madrid bombings."
Mr Dabas was arrested 24 hours after his brother Mohannad Almallah Dabas, a Syrian national, was arrested in Madrid.
The Spanish Interior Ministry has accused the two men of being linked to the recruitment of young Islamic radicals in Spain.
It claims they used a flat in Madrid to house recruits, or people passing through.
They are alleged to have been helped by a Syrian man, who has already been arrested and charged for his suspected role in the 11 March attacks.
The bombings have been firmly attributed to Islamic extremists by Spain's public prosecutor.
So far more than 70 people have been arrested, with 22 held on charges, over the bombings.
A Moroccan cell with links to al-Qaeda claimed responsibility for the attack, and most of those arrested have been Moroccan citizens.
The explosions came three days before a general election, in which the Socialists ousted the right-wing Popular Party (PP) of then Prime Minister Jose Maria Aznar.
Spain's government initially blamed armed Basque separatists Eta for the attacks, an error which is widely believed to have contributed to the Popular Party's unexpected defeat.
The PP is still convinced there could be a further link to Eta - and this has led to rancorous disagreement with the governing Socialists.