By Inti Landauro and Corina Pons

MADRID (Reuters) -Spain's Labour Minister Yolanda Diaz, who leads the ruling coalition's far-left junior party Sumar, called for a "reset" of government on Friday after a corruption investigation forced out a close aide of the prime minister.

Prime Minister Pedro Sanchez has apologised to Spaniards and promised an investigation of his Socialist Party but also resisted critics' calls for a snap election after Thursday's resignation of the party's number three Santos Cerdan.

A Supreme Court judge has asked Cerdan to testify over accusations, which he denies, of being involved in the awarding of public works contracts for kickbacks.

The fragile government has faced a series of scandals including a high-profile probe into whether Sanchez's wife Begona Gomez used her status to sway business dealings.

"This legislature needs an authentic reset," Diaz told reporters. "We will demand right now a meeting of the government coalition ... the only possible way out is a U-turn to take the legislature towards social policies."

Sumar pulls the coalition policies leftwards, pressing for less spending on defence and stronger worker protection, for instance.

"Forgiveness is not enough," Diaz added, without specifying further what she wanted the government to do.

Sanchez's coalition struggles to get any initiatives, such as budget bills, approved by lawmakers.

A sombre-looking prime minister said in his address late on Thursday that he regretted trusting Cerdan.

He took over as prime minister in 2018 after the biggest corruption investigation in Spain's democratic history netted scores of people linked to the then ruling People's Party, leading to the ejection of Prime Minister Mariano Rajoy.

Since then and after two elections in 2019 and 2023, Sanchez has ruled in minority coalitions.

(Reporting by Inti Landauro and Corina Pons; Editing by Andrew Cawthorne)