Maria Dolores de Cospedal, the party's former secretary general who served as defence minister between 2016 and 2018, and her husband, were accused of corruption, embezzlement and influence peddling.

She was also accused of trying to obtain information and documents from the PP's former treasurer, Luis Barcenas, who at the time was at the centre of a probe into a kickbacks scheme within the party.

Barcenas was later jailed for 33 years over the affair.

The aim of the alleged spying was to find out what dirt Barcenas had on party officials.

Quizzed as a defendant at the National Court in June, Dolores de Cospedal denied any involvement in the alleged spying on Barcenas and other party officials.

The court on Thursday shelved the case against her and her husband, citing "lack of evidence".

"The weakness of the reasons for incriminating them is striking," the court said in a statement.

By contrast, the court is expected to put former interior minister Jorge Fernandez Diaz on trial over the affair.

Both Dolores de Cospedal and Fernandez Diaz served under former PP prime minister Mariano Rajoy.

Last year, prosecutors said they were looking into widespread evidence the pair were involved in the spying scandal.

The probe into the so-called "Operation Kitchen" spying affair is one of several opened after the arrest of ex-police chief Jose Manuel Villarejo who for years secretly recorded conversations with top political and business figures in order to smear them.