The findings of a preliminary inquiry, launched in March, have provided enough possible evidence to open a formal investigation into "favouritism and profiting from favouritism", the Paris prosecutors' office announced on Friday.
Macron received a rapturous reception when he spoke to start-up operators at a "French Tech night" at the Consumer Electronics Show (CES) in the US resort on 6 January 2015.
He had been flown in specially and the event cost 381,000 euros, of which nearly 100,000 euros went in hotel bills.
By law any contract of more than 90,000 euros should be put out to tender but, although two other operators - Apco and Publicis - were contacted, no call for tenders was made.
Havas was awarded the contract in December 2015.
Auditor warned of legal danger
An audit by E&Y (previously Ernst and Young) in July 2016 warned there could be grounds for legal action, noting that "no purchase order, estimate, signed contract or bill" had been found.
A search of the offices of Havas and Business France, the state-run body that awarded the contract, on 20 June turned up an email from communications director Fabienne Bothy-Chesnau that appears to show that Pénicaud, who was Business France's boss at the time, was aware of the problems.
"Muriel, who has been briefed by us, is doing nothing," it read. "So she will handle things if the CDC (Court of Audit) demands an explanation [...] It won't be because it hasn't been mentioned and mentioned again."
Auditor refuses to validate contract
Pénicaud, who on Friday issued a statement "categorically" insisting she had done nothing wrong, claims to have "immediately" ordered both and internal and an external audit but, according to Libération newspaper, she did not inform her board of the problems until December 2016, nearly a year later.
And, the paper adds, that was only in the form of a few lines in Synthesis of audits made in 2016, stressing that the event took place despite short notice, while mentioning a "potential irregularity in the contractual services" that should be solved by a protocol that Havas had already signed.
Business France's own financial controller eventually refused to validate the contract, leading to Macron's successsor, Michel Sapin, notifying prosecutors in February 2017.
Macron collaborators worked for Havas
Sapin exonerated Macron and his collaborators but the investigation is now looking into the fact that several of Macron's colleagues at the economy ministry previously worked for Havas.
If the investigation finds "serious and concordant" evidence, Pénicaud could face charges, which according to rules established by Prime Minister Edouard Philippe, would oblige her to resign as the minister in charge of the government's top legislative priority, the reform of labour law.
Although the government is also preparing a bill to clean up French politics, Macron called on the media to "give up the ceaseless search for scandal, the permanent violation of the presumption of innocence" during his speech to both houses of parliament on Monday.
Four ministers resigned from the government before the reshuffle that followed last month's parliamentary elections because they were the subjects of legal inquiries.
One of them, Richard Ferrand, quit the interior minstry to lead the ruling party's parliamentary group.
Investigators questioned him on Thursday afternoon over accusations of favouritism when he worked for a mutual health insurance group.