A man being held in Switzerland is fighting extradition to Spain to face charges of planning a bomb attack on Madrid's highest court.
Swiss officials arrested Achraf in August on immigration charges
Swiss officials had agreed to an informal request for the extradition of Mohamed Achraf, said to be an Algerian leader of a militant Islamic group.
His decision to fight the request means delays as Spain must apply formally.
Mr Achraf is thought to have a planned to attack a Spanish court where judges are investigating the 11 March attacks.
Earlier this week, Spanish Interior Minister Jose Antonio Alonso said police had broken up what he called a radical Islamic cell involved in plotting the attack.
The plan is said to have involved detonating a truck loaded with 500 kg (1,100 lb) of explosives near the High Court and Supreme Court.
No weapons or explosives have been found but eight people have been arrested since Monday in connection with the investigation.
The suspects were said to include at least four Algerians and one Moroccan.
The interior ministry said the suspects had been in contact with other individuals in Europe, the US and Australia.
Spain's Deputy Prime Minister Maria Teresa Fernandez de la Vega said
Mr Achraf, 30, was wanted for crimes of "belonging to a terrorist group
and attempted terrorist murder".
"Mohamed Achraf is considered the suspected leader of the terrorist cell that was dismantled this week in Spain," she told a news conference on Friday.
It is seven months since Islamic militants launched bomb attacks on trains as they arrived in Madrid, killing 191 people.
Around 20 suspects are being held in custody because of alleged links to the attacks.
A further seven were killed when their explosives blew up as police approached their hide-out in Madrid.
The High Court handles cases of terrorism and is where the suspected Madrid train bombers are under investigation.