The head of Spain's Civil Guard was placed under formal investigation on Thursday for her alleged involvement in a plot to sabotage corruption probes targeting the ruling Socialists -- another blow to a fragile government beset by legal woes.

The alleged skulduggery is one of several scandals jeopardising the position of Prime Minister Pedro Sánchez, a global left-wing hero for his clashes with US President Donald Trump and Israel.

Former Socialist activist Leire Díez and disgraced ex-party heavyweight Santos Cerdán are suspected of leading a scheme to influence and destabilise sensitive legal investigations targeting Sanchez and his entourage.

The alleged plot included payments or favours for Civil Guard staff and prosecutors "in exchange for information or acts contrary to the performance of their duties", or finding compromising material against them.

Investigating judge Santiago Pedráz summoned Civil Guard director general Mercedes González and the police force's number two, Manuel Llamas, to testify as suspects on July 16, the Audiencia Nacional court said in a statement.