PARIS (Reuters) - France's prime minister, whose government has vowed to clean up politics, was forced on Wednesday to defend his decision to hire a private jet for 350,000 euros ($415,000) to fly back from Japan.

Opposition politicians decried Edouard Philippe's use of the private charter, and a non-governmental agency that pursues financial wrongdoing in high places accused him of ignoring his own government's pledges of exemplary behavior.

President Emmanuel Macron came under fire earlier this month for celebrating his 40th birthday in the grounds of a royal palace. His office sought to play that down, saying the event had been paid for by Macron and his wife.

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Philippe acknowledged on RTL radio that he and his delegation flew back to Paris from Tokyo, after an official trip to New Caledonia, at a cost of 350,000 euros, but said he had been obliged to do so.

"I totally understand the surprise and the questions of the French people," Philippe said. "We knew there was no commercial flight at the time we needed to return, and we knew we had to return because the President was leaving on the Wednesday morning of our return for Algeria," he said.

"The rule is that, whenever possible, (either) the Prime Minister or the President must be on national territory ... I take full responsibility for this decision."

Karim Bouamrane, spokesman for the Socialist Party, said on Twitter that the flight pointed to "amateurism" regarding the organizational skills of Philippe’s team.

Macron put financial and ethical probity in public life at the heart of a presidential election race he won last May, and his new government passed a law to tighten up on ethical standards in French politics.

He has struggled nonetheless to shake detractors' charges that he is a "president of the rich" after reforms including the scrapping of a wealth tax and reductions in housing benefit - moves Macron says will boost investment and social mobility.

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In August, social media commentators and political opponents criticized the French president after it emerged he spent 26,000 euros on makeup during his first 100 days in office.

Anticor, a non-governmental agency that focuses on financial corruption and profligacy in politics, said use of a private jet was at odds with Macron and his government's declarations that financial probity was a priority.

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