Spain's governing Socialists are to pass a law declaring the political courts that operated under dictator General Francisco Franco to be illegitimate, thus opening the way for thousands of sentences to be declared null, according to Spanish politicians.

Prime minister José Luis Rodríguez Zapatero's government has cleared the way for death sentences and other decisions of military tribunals and special courts to be challenged, Spain's communist-led United Left coalition group announced.

A new law will declare Franco's court martials, public order tribunals and courts set up specifically to pursue communists and Freemasons as "contrary to the law" and "illegitimate", said the United Left leader, Gaspar Llamazares. That would allow families of victims to ask the supreme court to declare their sentences null and, in turn, seek compensation, said Mr Llamazares. "This is a qualitative leap from impunity towards justice," he said.

Campaigners welcomed the agreement between the Socialists and United Left, who jointly see themselves as representing the republicans who lost the civil war against Franco's rightwing rebels. Campaigners said the new deal improved on an earlier Socialist proposal which shied away from annulling sentences and prevented the naming of those who administered Franco's political courts until his death in 1975.

The law would concentrate on the victims of Franco's nationalist side during the civil war rather than on the victims of the republic's mainly leftwing defenders, according to Mr Llamazares. Those killed in republican areas included more than 6,000 priests, monks and nuns.

Spain's rightwing opposition People's party accused Mr Zapatero, whose grandfather was shot by Franco's firing squad, of stirring up confrontation and of betraying a tacit agreement in democratic Spain not to rake over the coals of the civil war. "Parliament has never before been used to look back at that tragic and dramatic moment of history, the civil war," said parliamentary spokesman Eduardo Zaplana.

Mr Llamazares said the draft was not finalised, but that the United Left and the Socialists agreed on the fundamentals.