French conservative presidential candidate Francois Fillon's legal problems deepened on Tuesday, March 21, with financial prosecutors expanding a probe into payments to his family to suspected "aggravated fraud, forgery and use of forgeries", a judicial source said, according to AFP.

Investigators are probing whether Fillon and his wife Penelope forged documents to try to justify around 700,000 euros ($757,000) she earned for a suspected fake job as a parliamentary assistant, the source said.

The news came as Socialist Interior Minister Bruno Le Roux resigned after revelations that he had hired his two teenage daughters as parliamentary aides, prompting comparisons to Fillon's scandal.

France goes to the polls next month for the first round of a two-stage election to pick the next president. It has been a rollercoaster campaign, with a string of revelations that have knocked Fillon from the top of the opinion polls.

His wife Penelope and two of the couple's children are suspected of holding fake jobs as parliamentary aides for which they were paid around 900,000 euros in total.