A French prosecutor has opened an investigation into a close ally of President Emmanuel Macron, putting the new leader under growing pressure to clean up his own government after he campaigned to improve ethical standards in politics.

Mr Macron stayed silent after the announcement of the investigation into Territorial Cohesion Minister Richard Ferrand, an early ally who played a pivotal role in the presidential campaign.

The investigation comes at a bad time for Mr Macron and his government, as they are striving to get a majority in parliament in two-round legislative elections this month to push through his pro-business, pro-EU agenda - and just as Justice Minister Francois Bayrou detailed the first major bill of Mr Macron's presidency, a law intended to clean up corruption in politics.

Mr Ferrand is notably suspected of past business practices that benefited his romantic partner during the time when he led an insurance company.

After saying there were no grounds for an investigation, the prosecutor in Brest in western France reversed course and said it is opening a probe.

The U-turn follows media reports and questions about a possible conflict of interest before and after he became a legislator in 2012. Mr Ferrand has also acknowledged hiring his son as his parliamentary aide for a few months.

The prosecutor's statement said police will investigate whether there are grounds to potentially charge Mr Ferrand with property crimes, lack of probity and violating insurance codes.

Mr Ferrand acknowledged the facts in a statement this week, but denies any wrongdoing. Mr Macron and Prime Minister Edouard Philippe have stood behind him so far.

While visiting the western region of Brittany, a stronghold of his disputed minister, Mr Macron refused to answer questions from journalists. The prime minister stood by his line that Mr Ferrand can retain his position as minister as long as he is not given preliminary charges.

At this early stage of the judicial proceedings, Mr Ferrand cannot be charged. Only an investigating judge could hand Mr Ferrand preliminary charges, but the prosecutor has not yet decided to appoint one.

Even if Mr Ferrand is found to have done nothing wrong legally, his image is tainted and threatens to taint that of the new president. Mr Macron won the election in part on promises to renew French political practices and figures from top to bottom.

Calls for Mr Ferrand's resignation or dismissal have come from all political sides and also from civil society.

France's leading anti-corruption association, Anticor, urged the minister to resign in order to "not interfere" with Mr Macron's efforts to "moralise" political life.

"Whatever the outcome of the judicial inquiry," Anticor's president, Jean-Christophe Picard, told BFM television, "his political accountability is already at stake."

From the far-right to far-left of the political spectrum, several politicians stressed that the minister now has no choice other than to resign, saying the credibility of the government in its fight for higher standards of transparency and ethics is at stake.

Far-right leader Marine Le Pen said that "there has been at least a moral breach" in Mr Ferrand's case. Conservative legislator and former magistrate Georges Fenech said that "if Macron doesn't get Ferrand's resignation within the hour, he will no longer be credible".

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