Bush and Aznar were strong allies in the run up to the war in Iraq

The decision, which makes the group liable to sanctions in the US, came after pressure from Spanish Prime Minister Jose Maria Aznar's, currently in Washington for talks with President George W Bush.

Mr Aznar was a staunch ally of Mr Bush's in the run-up to the war against Iraq, supporting US moves to win United Nations Security Council backing for military action.

Batasuna is already permanently banned in Spain on the grounds that it is part of the "terrorist network" of ETA - a charge Batasuna members vehemently deny.

The ban, which was upheld by Spain's supreme court in March, was the first ban on a political party since the death in 1975 of dictator General Francisco Franco.

Visa ban

US Secretary of State Colin Powell signed the order adding three Basque nationalist groups - Batasuna, Euskal Herritarrok and Herri Batasuna - to the US list on 30 April.

Batasuna could now be subject to sanctions in the US

He agreed with Spain's claim that the three organisations are fronts for the guerrilla group ETA, which is already on the state department's list of "foreign terrorist organisations".

Mr Powell signed the decree just one day before he visited Madrid, but its announcement was delayed, according to usual practice, to ensure that the organisations named would not have time to dodge financial sanctions by placing their funds out of reach of US authorities.

This latest decree is directly linked to an executive order issued by Mr Bush in the wake of the 11 September attacks in the US, which prohibits all financial transactions, including donations, with groups placed on the terrorist list.

Ordinarily members of groups placed on the list are also subject to a US visa ban.

Mr Aznar has long been pressing for action against groups linked to ETA.

ETA - whose full name Euskadi Ta Azkatasuna stands for Basque Homeland and Freedom - first emerged in the 1960s as a student resistance movement bitterly opposed to General Franco's repressive military dictatorship.

In subsequent decades the armed organisation has waged a bloody campaign for independence for the seven regions in northern Spain and south-western France that Basque separatists claim as their own.