Luis Barcenas is a former senator, and was the treasurer of the Popular Party from 1990 to 2009.

BBC: Q&A: Spain's opposition tells PM to go

Money was allegedly paid by outside firms via two former treasurers including Luis Barcenas, who stepped down in 2009 and is currently under investigation for money-laundering.

BBC: Spain ruling party faces inquiry over 'slush fund'

El Pais is adamant that the ledgers, dating from 1990 to 2008, were written by the party's former treasurer, Luis Barcenas who stepped down after being implicated in a separate, high-profile corruption case.

BBC: Spain's PM Mariano Rajoy pledges to battle on