Public disillusion in established politicians helped Macron win the presidential election campaign.
French political life has been hit by a succession of corruption scandals - not least the fake-jobs allegations that hit mainstream-right candidate François Fillon during the presidential campaign - and Macron promised a law to enforce higher ethical standards.
Interior Minister François Bayrou has been charged with delivering it as one of the new government's top priorities.
Now, just a week after the new government was formed, the Canard Enchaîné weekly has revealed that the partner of Territorial Cohesion Minister Richard Ferrand was awarded a highly profitable contract by a group of mutual insurance companies while Ferrand was its boss.
Ferrand, a Socialist MP who defected to the Macron camp, was a high-profile campaigner during the election campaign after being appointed secretary general of the presidential hopeful's En Marche ! movement.
Partner's company chosen
Ferrand was managing director of Les Mutuelles de Bretagne from 1993 to 2012, helping save the group of mutual insurance companies from serious financial difficulties.
In 2011, according to the Canard Enchaîné, the group agreed to rent premises from a property company owned by Ferrand's partner, Sandrine Doucen, for a rent of 42,000 euros a year.
The company, Saca, was chosen from three bidders, even though it had not yet been legally established and did not yet own the premises, in the centre of the Brittany city of Brest.
Les Mutuelles de Bretagne also agreed to renovate the offices at an eventual cost of 184,000 euros.
The Canard Enchaîné accepts that the operation was legal but points out that the deal proved very profitable for Doucen, who is a lawyer by profession.
Once Saca had been legally registered, with a capital of 100 euros - 99 of which were provided by Doucen the other by a family friend - the contract enabled it to win a 402,000-euro loan to buy the property.
The value of its shares has since risen 3,000 times, according to the paper, and the one-euro share has been sold to one of Ferrand's and Doucen's daughters.
Ministry cries slander
A statement by Ferrand's ministry on Wednesday described the report as "slanderous denunciations that have been pursuing M Ferrand for years".
Ferrand was not a member of the board of directors that took the decision, the statement said, adding that the board knew full well that Saca belonged to his partner, a claim that Ferrand himself made to the Canard Enchaîné.
But the chairman at the time, Michel Buriens, told the paper's reporters he could not remember if that was the case.
Government spokesman Christophe Castaner praised Ferrand as a man of "exceptional probity".
"One thing is certain, there was nothing illegal, there was nothing that was not moral," he told Europe 1 radio.
Sources in Prime Minister Edouard Philippe's office told the media that there was no question of Ferrand resigning.
Calls to resign
Socialist Party national secretary Jean-Christophe Cambadélis seemed unconvinced, however.
He called on Philippe and Bayrou, a long-time transparency campaigner, to issue statements on the question, while the Young Socialists called on Macron to fire the minister.
National Front leader Marine Le Pen, who lost to Macron in the presidential deciding round, called on Ferrand to resign.
The European parliament is currently deducting a monthly sum from her salary to reimburse payments for what it says were fake jobs for party members.
The Canard Enchaîné also reports that Ferrand employed his son as a parliamentary assistant for several months for a salary of 8,704 euros.