The lawyers said the two-week-old probe into the payments to Penelope Fillon, who was hired by her husband as a parliamentary aide, was “invalid” and “completely tramples democratic principles”.
The investigation dealt a “serious blow to the principle of the separation of powers” between the judiciary and the legislature, lawyer Antonin Levy told a press conference.
Levy and fellow lawyer Pierre Cornut-Gentille argued that Fillon enjoyed discretion in the use of funds available to him as an MP and that the probe into misuse of public money was therefore “totally inapplicable”.
“The absence of such an offence makes the financial prosecutor’s office incompetent to investigate and the investigation is therefore illegal,” Levy said.
He also criticised constant leaks about the investigation to the media which he said were damaging to his client and likely to have an impact on the two-stage presidential election in April and May.
“The financial prosecutor’s office has a responsibility. It’s important that voters don’t have their election stolen in April,” Levy added.
Fillon’s presidential bid has been thrown into turmoil by the revelations that his wife was paid hundreds of thousands of euros over 15 years for a role she is suspected of not having fulfilled.
The conservative former prime minister, who had been leading the race before the story broke, has denied any wrongdoing.
His popularity ranking has tumbled from third to 18th position over the past month, according to a poll by Elabe for Les Echos newspaper yesterday.
The poll gave Fillon a rating of 22%, down 13 percentage points from a month ago.
Fillon has been replaced as favourite to become president by centrist Emmanuel Macron, according to polls of voting intentions.
Macron is the most popular politician in the Elabe popularity poll, up two percentage points on a month ago at 43%.
Socialist Benoit Hamon, who won his party’s presidential ticket last month, saw his popularity rating climb 10 points to 37%.
National Front leader Marine Le Pen, seen by polls of voting intentions as likely to lose to Macron in a deciding presidential vote on May 7, saw her popularity fall by one point to 28%.
Meanwhile, according to an Opinionway poll yesterday, Macron would beat Le Pen in the run-off vote for the French presidential election.
The latest Opinionway poll, which is being conducted daily in the run-up to the election, showed Le Pen would get the biggest score in the first round of voting on April 23.
Le Pen would get 24% in this first round, compared to 21% for Macron and 20% for Francois Fillon, the candidate of The Republicans’ right-wing party.
Macron would go on to beat Le Pen in the May 7 run-off vote, with 65% to 35% for Le Pen, the poll showed.