A Spanish judge has withdrawn from the murder trial of four members of the armed Basque group ETA after an open microphone in court caught her calling them "bastards."

Judge Angela Murillo, who has been accused of bias in previous ETA trials, made the comments on Wednesday during the trial of the four for the 2001 murder of politician Jose Javier Mugica.

"I have decided to withdraw from this case in order not to jeopardize the proceedings, the parties involved, the defendants and the victims," Judge Murillo was quoted as saying by national radio on Thursday.

ETA called an end to its 40-year campaign of bombings and shootings in October, but any reconciliation between the armed group and their victims is a long way off.

Referring to Mugica's wife, who was in the courtroom to give testimony about the events leading up to her husband's murder, Murillo said: "Poor woman. And on top of everything, those bastards are laughing."

In February, the Supreme Court overturned a two-year sentence Murillo handed to the former head of ETA's political wing Arnaldo Otegi, after ruling that a comment she had made exposed her bias.

The judge had said: "I knew you weren't going to respond," when the defendant refused to answer a question.