This was alleged in January 2013. According to at least one Spanish newspaper, President Rajoy and several other high ranking politicians received secret payments outside their official salaries. It is said that the President's Popular Party operated a secret fund or 'slush fund' from which they made illegal payments. Rajoy and others accused of this have publicly and consistently denied the allegations. So far nothing has been proved and no legal action has been brought against the government, and if this continues they may survive the scandal.
However, despite the lack of proof many people in Spain believe the allegations. According to polls the level of public trust in governments and politics is extremely low; most people believe politicians of all parties are involved in corruption, and do not believe that Rajoy or anyone else is likely to solve the country's financial crisis. The fact the he has refused to have an inquiry into the allegations has not helped his public standing; as for whether the rumours are true, only time will show.