Embattled French presidential hopeful Francois Fillon received an undeclared loan of $53,000 from a billionaire friend in 2013, according to the Canard Enchaine weekly.

The conservative candidate "did not deem it necessary" to report the interest-free loan from Marc Ladreit de Lacharriere to a state transparency watchdog, the weekly said in its edition to appear on Wednesday.

The investigative and satirical newspaper reported that Fillon's lawyer Antonin Levy had confirmed the loan had been repaid in full.

"The 'oversight' may be costly for the presidential candidate," said the publication.

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Ladreit de Lacharriere is the CEO of Fimalac, a financial services holding company, and owns the literary magazine La Revue des Deux Mondes, which employed Fillon's wife, Penelope.

The publication paid Penelope Fillon some $105,000 euros in 2012-13, but there is little evidence of her work.

Investigators are also looking into a possible link between this job and the bestowal of France's highest civilian honour, the Grand Croix of the Legion of Honour, on Ladreit de Lacharriere in 2011 when Fillon was prime minister.

The Canard Enchaine also said investigators were also looking into a consultancy firm called 2F Conseil that Fillon set up in 2012 after he left office as prime minister, which the paper says has paid him hundreds of thousands of euros.

Fillon has denied any wrongdoing with his consultancy work.

The 63-year-old former prime minister faces embezzlement charges later this month.

'Waste' candidacy

Once the frontrunner to become France's next president in May, he has had to battle to stay in the race because of revelations in January that he had paid his wife hundreds of thousands of dollars from public funds, allegedly for fake jobs.

With just seven weeks to go before France goes to the polls in the April 23 first round of a two-stage vote, Fillon has remained as the rightwing candidate despite calls for him to allow rival Alain Juppe to stand in his place because of the scandal.

On Monday, he won the "unanimous" backing of his Republicans party after Juppe, 71, adamantly rejected the calls while lamenting the state of Fillon's candidacy, calling it a "waste".

Juppe also chastised Fillon for the disdain he has shown the justice system and his swipes at the media.

The infighting among Republicans and Fillon's chaotic campaign have made an already unpredictable election even harder to call.

Centrist candidate Emmanuel Macron, 39, has gone from strength to strength, with the latest poll putting him neck-and-neck with far-right leader Marine Le Pen with 25.5 percent to 26.5 percent for her.

Fillon is lagging behind in third place with 18.5 percent, according to the Ifop-Fiducial poll out on Tuesday.