Romanian prosecutors said on Wednesday they had opened a criminal investigation into former president Traian Basescu over threats he made to a senator while still head of state.

Last year Basescu warned leftist senator Gabriela Firea on television to mind her own business and to take care of her husband, Florentin Pandele, the mayor of a small Romanian town near the capital Bucharest.

Basescu, who is from the center-right, then said Firea "might not find him at home one day if she is not careful."

Firea, then spokeswoman for leftist Prime Minister Victor Ponta's failed 2014 presidential campaign, said the comments were "clearly aimed at threatening a senator and humiliating a citizen of this country" and she filed a complaint.

Basescu, an outspoken former sea captain who served as president from 2004 to 2014, lost his immunity from prosecution after stepping down as head of state.

"In the case that started as a result of the complaint filed by Gabriela Firea ... prosecutors have proceeded with a criminal investigation on suspicion of committing blackmail," the Prosecutor General's office said in a statement.

"Traian Basescu came to the prosecutors' headquarters today to be notified of the charges and his defenders have requested a deadline to study the file."

Basescu declined all comment. He has previously called Firea a "blackmailer".

He has a track record of inflammatory comments and accusations. In 2007, in a private conversation with his wife that was recorded, Basescu called a reporter a "filthy gypsy" and was reprimanded by the national anti-discrimination office.

The case is also the latest in a series of setbacks for Basescu, whose brother is under investigation for allegedly taking a bribe to help keep an underworld boss out of jail.

His former protege and presidential candidate Elena Udrea is also under investigation for graft amid an ongoing crackdown on high-level corruption. Udrea has denied wrongdoing.

In a separate case, prosecutors are also looking at the circumstances under which one of Basescu's daughters purchased farm land.