By Luiza Ilie and Radu-Sorin Marinas

BUCHAREST (Reuters) - Romanian prosecutors have widened a probe into former finance minister Darius Valcov, accusing him of making unlawful business gains after discovering cash, gold bars and a French impressionist painting in his safe.

Valcov, the most senior sitting politician in the country to have been investigated for corruption in an ongoing crackdown on high-level graft, told Reuters that he was innocent and would explain his actions in parliament.

Prosecutors launched an investigation into Valcov earlier this month, accusing him of favoring a firm for a public works contract in exchange for about 2 million euros ($2.20 million) while in his former post as mayor of the city of Slatina.

Valcov, who is also a senator, resigned as finance minister days later, and Prime Minister Victor Ponta currently serves as interim finance minister.

Parliament is due to meet on Wednesday to decide whether to agree to a request from prosecutors to issue a temporary arrest warrant for Valcov, pending the investigation.

Under Romanian law, prosecutors need parliament's approval to investigate and detain sitting lawmakers for alleged graft offences committed while they were in office.

"Beginning with 2011 and until the present, Valcov conducted financial operations or business deals incompatible with his post as mayor, senator and minister ... concerning land registry, accounting and law firms which he owns and manages through third parties," prosecutors said in a statement.

They added that some of the gains squirreled away by Valcov included a lump sum of $90,000 (81,960 euros), three gold bars weighing a total three kilos (6.6 pounds), and three paintings -- all found in a safe.

One of the paintings appeared to be by the French master Pierre-Auguste Renoir.

Parliament has a track record of rejecting prosecutors' requests and has blocked probes into two politicians so far this year. The senate's legal commission on Tuesday voted against allowing the arrest of Valcov, although its recommendation is not binding.

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Romania is seen as one of the European Union's most corrupt states, but its prosecutors and magistrates have won praise from Brussels for crackdowns that have seen many members of parliament face trial for graft-related offences.

"I can't comment more, but I can only say I am innocent and will try to prove my innocence until the very end," Valcov told Reuters by telephone. "I will explain this in parliament."

Prime Minister Ponta's government is expected shortly to approve proposals for sweeping tax cuts from 2016-2019 that could be sent to parliament this week.

(1 euro = $1.0981)

(Editing by Matthias Williams and Crispian Balmer)