Spain's supreme court Thursday overturned the convictions of four of 21 people condemned to prison over the Madrid train bombings that killed 191 people in 2004. It also upheld a lower court's decision to acquit one of the alleged masterminds of the Al Qaeda-inspired attacks, Rabei Ousmane Sayed Ahmed, known as "Mohammed the Egyptian".

The early morning bombings on four packed commuter trains on March 11, 2004 were the deadliest terror attacks in the West since the September 11, 2001 strikes against the US.

On October 31 last year, Spain's anti-terrorist court convicted 21 people of involvement, and acquitted six others.

The Supreme Court Thursday overturned the convictions of Basel Ghalyoun and Mohamed Almallah Dabas, both condemned to 12 years in prison for belonging to a terrorist group.

It also absolved Abdelilah El Fadual El Akil, condemned to nine years for collaborating with a terrorist group, as well as Raul Gonzalez Pena, who had received five years for supplying explosives.

But it rejected an appeal by prosecutors against the acquittal of "Mohammed the Egyptian".

The court had ruled last October that there was not enough evidence to condemn him as an organiser of the attacks.

But prosecutors had only appealed his acquittal on the charge of belonging to a terrorist group.

On that charge, the judges in October decided that he could not be convicted twice for the same crime, as he was already serving an eight-year prison sentence in Italy for belonging to a terrorist group.