Madrid.—Spain's prime minister published his tax returns on Saturday in a bid to quell reports he and other conservative politicians received secret cash payments but the opposition said many questions remain unanswered.

The government's website posted tax authority documents detailing Mariano Rajoy's income and tax payments from 2004 to 2012. His ruling People's Party (PP) also revealed four years of financial accounts on Friday, in another attempt to put the matter to rest.

The scandal, centred on ledgers supposedly made by a former party treasurer, have cut support for the PP to the lowest level on record and pushed up borrowing costs just as it seemed Spain was getting to grips with a financial crisis that had raised questions about the future of the euro zone.

Former PP treasurer Luis Barcenas has described as fake handwritten ledger entries published on Jan. 31 by El Pais newspaper purporting to show payments funded by construction firms made to PP leaders including Rajoy. Rajoy has said the payments were not made and that the party is organising an external audit into the affair.

The published documents showed Rajoy earned up to 147'620 euros ($197'500) per year 2004 to 2012 as a parliamentarian over the period plus income from investments.