PARIS (Reuters) - French President Emmanuel Macron’s promise of clean politics suffered a setback on Friday when the Paris prosecutor opened an investigation into his alliance partner, the centrist Modem party, over allegations of misuse of public funds.

FILE PHOTO: French President Emmanuel Macron attends a ceremony at the Arc de Triomphe in Paris, France, June 3, 2017. REUTERS/Charles Platiau

The probe threatens to cast a shadow over the first round of a parliamentary election on Sunday, though it and other misconduct allegations appear not to have hurt Macron’s Republic on the Move (LREM). Opinion polls show the party on course to win a landslide majority.

The prosecutor’s office said the preliminary investigation follows a letter received from a former Modem employee who said he was paid in 2010-2011 in part by parliamentary funds for work he carried out for the party.

The affair is awkward for Macron, who has picked two ministers from Modem, Justice Minister Francois Bayrou and European Affairs Minister Marielle de Sarnez.

Earlier on Friday, Prime Minister Edouard Philippe said any minister put under formal investigation for improper conduct must resign. A formal investigation follows a preliminary inquiry if the prosecutor sees grounds for a full-blown probe.

“I don’t think his position is weakened,” Philippe said of Bayrou on Europe 1 radio.

On Thursday, Modem said in a statement that it had “respected all the rules and all the obligations of an employer.” It did not immediately react to news of the investigation.

In March, the Paris prosecutor began a preliminary investigation into 19 European parliament lawmakers, including de Sarnez, following a complaint from an opponent. De Sarnez denies any wrongdoing.

An Ipsos Sopra Steria poll published on Friday found LREM was poised to win nearly a third of the vote in the first round on Sunday, setting it up to secure the biggest majority in nearly fifty years.

The election takes place on June 11 and 18.