MADRID (Reuters) - Spain's Socialist leader said on Monday his party would not back acting Prime Minister Mariano Rajoy's re-election and end an eight-month political impasse after meeting him for the final time before a confidence vote in parliament on Aug. 31.

"It is Mr Rajoy's responsibility to reach the 176 votes (he needs), exclusively Mr Rajoy's and not the Socialists'," Pedro Sanchez told reporters.

Rajoy's center-right People's Party is seven seats short of the majority it needs in the vote after reaching an agreement with centrist party Ciudadanos on Sunday. The Socialists' abstention would be enough to enable a PP-led minority government, but so far they have refused.

(Reporting by Angus Berwick; Editing by Catherine Bennett)