BRASILIA (Reuters) - A Brazilian federal prosecutor has opened an investigation into allegations that former President Luiz Inacio Lula da Silva was involved in a vote-buying scheme in Congress that led to the conviction of close aides for corruption.

The federal prosecutor's office said in a statement late on Friday it asked the federal police to probe accusations of a businessman at the center of the corruption case, Marcos Valerio, who alleged that Lula not only knew about the illegal scheme but received money from it.

Lula has repeatedly denied the allegations that he knew of the vote-buying scandal known as the "mensalão," under which operatives from the ruling Workers' Party paid lawmakers in Congress to back the government's legislative agenda. The scandal, which erupted in 2005, almost brought down Lula's government at the time and led to the biggest political corruption trial in Brazilian history.

Lula led Brazil from 2003 and 2010 and is a powerful political figure in Latin America's largest country. He remains highly influential in the government of the current president, Dilma Rousseff, his political protégé and former chief of staff.

Although the immensely popular Rousseff has so far avoided any political fallout from the years-old scandal, that could change if Lula is found to have been directly involved. She plans to run for re-election next year.

In testimony last September, Valerio alleged that Lula was aware of the vote-buying scheme and that some of the former president's personal expenses were paid with cash from illegal campaign funds. Valerio, an advertising executive who worked closely with Lula's Workers' Party, gave the testimony in a bid to reduce his sentence after he and 24 other former Lula aides and associates were convicted in a landmark trial heard by Brazil's Supreme Court.

The circumstances of the testimony, and Valerio's own conviction to 40 years in prison on corruption charges, have raised questions about the credibility of his accusations.

A spokesman of Lula's foundation contacted by Reuters on Saturday declined to comment on the news of the investigation.

It was not clear how long the investigation could last, but probes in Brazil are known to go on for months and in some cases several years. Some legal experts have voiced doubts that Lula will ever be formally charged with a crime.

($1 = 1.9961 Brazilian reais)

(Reporting by Alonso Soto and Maria Pia Palermo; Editing by Vicki Allen)