The scandal-hit French presidential candidate François Fillon received an interest-free, undeclared loan of €50,000 (£43,000) from a billionaire businessman in 2013, according to the latest revelations by the French weekly Le Canard Enchainé.

Fillon “did not deem it necessary” to report the loan from the French business tycoon Marc Ladreit de Lacharrière to a state transparency watchdog, the paper said. Fillon’s lawyer told the paper that he had fully repaid the loan. The paper said Fillon explained to investigators that he had simply forgotten to mention the loan in his declaration.

Five weeks ago, Le Canard Enchaîné broke the biggest political scandal of the French presidential campaign when it said that Fillon had allegedly secured his wife generously paid, taxpayer-funded “fake jobs” as a parliamentary assistant over 15 years. He was also reported to have secured similar jobs for his children. Fillon, the presidential candidate for the rightwing party Les Républicains, is now the subject of a full investigation by judges on several possible charges. He denies breaking the law.

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The latest allegation about the undeclared loan highlights another aspect of Fillon’s current legal woes: the relationship between the Fillon family and Ladreit de Lacharrière. The businessman is the chief executive of Fimalac, a financial services holding company, and owns the literary magazine, La Revue des Deux Mondes.

Among the issues currently being investigated by judges is Fillon’s alleged failure to make a full and complete declaration of his financial situation to the state transparency watchdog for people in public office. Fillon has denied any wrongdoing. “This loan existed, it was paid back, it’s a non-event,” a lawyer for Ladreit de Lacharrière told AFP.

Fillon is also under investigation over misuse of company funds after his wife, Penelope, was paid large sums by the literary review owned by Ladreit de Lacharriere, for which she was alleged to have only written two book reviews. Fillon has denied any wrongdoing and insisted his wife’s consultancy job for the review was real.

Judges are looking at whether Ladreit de Lacharrière paid Penelope Fillon about €5,000 a month before tax between May 2012 and December 2013 in return for being recommended by Fillon for France’s highest honour, the Grand Cross of the Legion of Honour, in 2010. This could be classed as influence-peddling.

Le Canard Enchaîné 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.

Since Fillon announced last Wednesday that judges had summoned him to press charges later this month, and made a speech attacking the justice system and the media, his party has been in turmoil over his troubled presidential campaign, with over 300 elected politicians storming out.

Fillon has refused to step down, insisting he will continue as candidate.

Fillon has dipped in the polls, lagging behind the far-right Front National’s Marine Le Pen and the independent centrist Emmanuel Macron in the April first round vote. The final runoff takes place on 7 May.