For supporters, Castex is a hugely impressive bureaucrat who will rapidly master the brief of prime minister and handling relations with President Emmanuel Macron, who sits atop a presidential system where the premier is very much number two.

But detractors have already rubbished the appointment, asking why Macron has bothered to replace Edouard Philippe, also a right-winger who has served for three years, with more of the same.

Castex, 55, mayor of Prades, a small town in the southern Pyrenees mountains, is no stranger to the corridors of power since his time as an advisor to ex-French president Nicolas Sarkozy.

"A senior civil servant who knows the health world perfectly well and who is impressively efficient," said the now former premier Philippe when Castex was nominated to design the easing of the coronavirus lockdown.

France's gradual easing of the lockdown has been regarded largely as a success, with life returning to normal across the country but with no sign of the feared "second wave" yet.

Castex was twice chief of staff for Xavier Bertrand, who was health minister and then labour minister under the presidencies of Jacques Chicac and Sarkozy.

At the time Castex had to deal with a number of sensitive cases, such as a pension reform and a law which forced strikers in the transport sector to provide a minimum service. 

"His warmth and charm are incredible and he is naturally humble and empathic," said a colleague who worked with Castex when he was head of the health cabinet, who asked not to be named.

"I've never seen anyone with such unanimous support all the time," said the same source.

Castex's politics are resolutely right-wing and has said in the past that his is "completely comfortable with that fact."

Father of four daughters, Castex was until January the coordinator between ministries in preparation for the 2024 Paris Olympic Games and head of the National Sports Agency. 

"He's a jack of all trades, he has connections everywhere and he knows what to do at the right time and in the right place," said ex-advisor to Sarkozy Franck Louvrier. 

Xavier Bertrand, now head of France's northern Hauts-de-France region, tweeted that "I know and I appreciate the qualities" of Castex as "a servant of the state." 

"They will be essential in the difficult times that we are going to face," he said.

But left-wing MEP Manon Aubry was dismissive. "Everything changes so that nothing changes! A man of the right replaces a man of the right to pursue the same anti-social and anti-ecological policies," she said on Twitter.

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