The inquiry, ordered by prosecutors in the western French city of Brest on Thursday morning, was "good news", according to government spokesman Christophe Castaner, since it would "allow us to put an end to the moral debate".
"It will allow us to put an end to people reading articles in the press that cast aspersions on Richard Ferrand and to leave things to the people who have the ability and power to deal with the case," he added.
Echoing an earlier statement by Prime Minister Edouard Phlippe, Castaner said that if Ferrand is charged he will be fired from his minister's job.
President Emmanuel Macron, who was visiting Brittany on Thursday, declined to comment on the latest development.
Possible breaches of law
The investigation was launched after anti-corruption campaign Anticor filed a complaint targeting Ferrand, the board of directors of the group he headed, Les Mutuelles de Bretagne, and his partner, Sandrine Doucen.
Prosecutors will seek to establish whether there has been a breach of rules governing mutual insurance companies, which are owned by their subscribers, of the kind that belong to Les Mutuelles de Bretagne.
Earlier in the week the Brest prosecutors' office refused to open an inquiry.
Ferrand, the government and the insurance group's current management all insist that the deal was legal and dismiss other accusations of dubious practice.
But Anticor argues that, if the deal is found to have been concluded in Doucen's interests rather than those of the subscribers, that would constitute breach of trust and personal enrichment.
Other legal questions raised include the possibility that a statutory auditor should have been notified of a potential conflict of interest and that the Mutuelles should not have awarded a contract that enabled Doucen to obtain the loan to buy the premises in question.
Clean-government bill undermined
The controversy has exploded as the government prepares a law to clean up political life, one of Macron's key promises during the presidential election campaign.
Interior Minister François Bayrou is to present it in full on 14 June and has been meeting party leaders this week.
Politicians of left and right criticised Macron and his government on Thursday:
Xavier Bertrand, a mainstream right-winger who refused to join the Macron government, judged the clean-government bill discredited and the president's attitude "inconsistent";
Raquel Garrido, of Jean-Luc Mélenchon's hard-left France Insoumise, said the affair reveals a "culture of impunity" for politicians in a "presidential monarchy";
Far-right leader Marine Le Pen declared that it is "strangely similar" to the fake-jobs scandal that torpedoed Republicans candidate François Fillon's presidential bid.