British police arrested on Saturday a man wanted by Spanish police in connection with the Madrid train bombings that killed 191 people a year ago.
Mr Moutaz Almallah Dabas, 39, was arrested by British extradition and anti-terrorist police in Slough west of London and is due to appear at Bow Street Magistrates Court on Monday, a police spokeswoman said.
"He was arrested under a European arrest warrant which was issued by Spanish authorities for alleged terrorist offences, in particular the involvement in the Madrid bombings," she said.
His arrest follows the arrest on Friday in Spain of his brother Mr Mohannad Almallah Dabas.
Spanish police said Mr Mohannad was Syrian, but British police said Mr Moutaz held Spanish nationality.
Both men are accused of training young Islamist militants at their home in Madrid and sending them overseas, the Interior Ministry said in a statement.
Investigators allege Mr Mohannad cooperated with another Syrian, Mr Basel Ghalyoun, who is in prison accused of participating in the train bombings.
Al Qaeda has been linked to the bombings on March 11, 2004 in which 10 bombs exploded on four packed morning commuter trains three days before a general election.
Police say the Almallah Dabas brothers associated with a man identified by investigators as a leader of the bombers: Mr Serhane ben Abdelmajid Farkhet, also known as "The Tunisian."
Mr Farkhet was one of seven suspects who died on April 3 when they blew themselves up rather than surrender to police who cornered them in an apartment in a Madrid suburb.
The train attacks helped to sweep Spain's Socialist party to power amid voter anger at the previous conservative government's support for the US-led war in Iraq and its attempts to blame the bombings on Basque separatists ETA despite evidence of Islamist involvement.