It comes as Mr Cahuzac admitted to having had a foreign bank account.
The ex-minister had until now denied a report by investigative website Mediapart that he kept an undeclared Swiss bank account until 2010.
Mr Cahuzac - who gained a reputation as a vocal crusader against the use of overseas tax havens - quit in March.
Mr Cahuzac said he made the admission about the bank account during a meeting with investigating magistrates on Tuesday.
In a statement, he said he had about 600,000 euros (£509,000; $770,000) in the account.
He also asked for forgiveness from French President Francois Hollande and his former government colleagues.
"I was caught in a spiral of lies and lost my way. I am devastated by guilt," the statement said, quoted by the AFP news agency.
"It was an unspeakable mistake to think that I could avoid confronting a past that I wanted to consider behind me. I will now face this reality with all transparency," the statement said.
Mr Cahuzac had earlier insisted that he had done nothing wrong. He was a plastic surgeon before entering politics.
Correspondents say the scandal is embarrassing for the French Socialist government which has made fighting tax evasion a priority, as it tries to cut public debt.

It happened at HM YOI Aylesbury in Buckinghamshire on Friday morning, Thames Valley Police said.
An online prison blog said 30 inmates took part in the "mass brawl".
Police said the injured people had since been released from hospital. The Prison Service said it was investigating the incident with police.
Officers were called to "a report of disorder" at the young offender institution in Bierton Road at about 11:00 BST, but "officer deployment was not required at the incident", a spokeswoman said.
"Seven prison officers and one prisoner were taken to hospital for treatment."
The incident came to light in an online blog on Prison UK.
Author Alex Cavendish, a former prisoner, said he was passed the information via "reliable, professional sources inside the system".
"A mass brawl broke out yesterday morning... Prisoners were attacking each other with weapons - and staff," the informant said.
He described the incident as "horrific".
"Staff were trying to save their lives and got battered. Some were seriously injured. Another officer was on the landing unconscious.
"We had ambulances and fire service in, trying to help alongside our healthcare department.
"The wing has been brewing for a while," he added.
A Prison Service Spokesman confirmed that "an incident involving a number of prisoners took place on Friday 21 July".
"We do not tolerate violence against our hard-working staff. Where incidents like this occur, we will always work closely with the police to push for the strongest possible punishment," he added.
The incident is being jointly investigated by the Prison Service and Thames Valley Police.
Former French Budget Minister Jerome Cahuzac has been placed under formal investigation over alleged tax fraud, his lawyer has said.
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Seven prison officers and one prisoner were taken to hospital after "disorder" at a young offender institution, police have said.