The King has branded global warming “our most existential challenge” a day after Prime Minister Rishi Sunak scaled back key green efforts to tackle the climate crisis.
Charles made the remarks in a historic address during his state visit to France as he became the first British monarch to speak in the French senate chamber.
He called for a new Entente Cordiale designed to tackle the global climate emergency, and stressed the need to “stride forward with hope and courage”, with his 18-minute long speech – prompting a standing ovation.
French President Emmanuel Macron later paid tribute to the King’s long-standing “conviction” and “preaching” on tackling climate change, as the pair visited a sustainability reception for business leaders and ministers at the Natural History Museum in Paris.
Mr Sunak has faced fierce criticism after weakening a host of pledges on Wednesday designed to help the UK reach net zero by 2050.
He pushed back a ban on new petrol and diesel cars by five years and watered down the plan to phase out gas boilers by 2035, but has insisted he is “absolutely not slowing down” efforts.
The monarch, as the Prince of Wales, spent decades passionately campaigning on green and environmental issues.
The King, switching between French and English, pledged to do whatever he could to strengthen the “indispensable relationship” between the UK and France.
He told of the “friendship and warm familiarity” between the two countries and the UK and France’s solidarity over the war in Ukraine, but urged the countries to work together on tackling climate change.
“Just as we stand together against military aggression, so must we strive together to protect the world from our most existential challenge of all – that of global warming, climate change and the catastrophic destruction of nature,” he said.
He suggested the Entente Cordiale – an agreement signed between UK and France in 1904 to set aside rivalries – be renewed as an Entente pour la Durabilite (Sustainability Agreement) to deal with the global climate emergency.
The King said: “For the time that is granted to me as King, I pledge to do whatever I can to strengthen the indispensable relationship between the United Kingdom and France – and, today, I invite you to join me in this endeavour.
“Together, our potential is limitless. Let us, therefore, cherish and nurture our Entente Cordiale.
“Let us renew it for future generations so that, I would like to propose, it also becomes an Entente pour la Durabilite – in order to tackle the global climate and biodiversity emergency more effectively.
“A commitment to each other, and to the values we so proudly share; a commitment inspired by the example of the past, and emboldened to grapple with the immense challenges in the world around us.
“As neighbours, friends, partners and allies, there is no challenge to which we cannot rise, as we have done so often in the past. Let us stride forward with hope and courage – and let us do so together.”
Charles opened his speech in French, thanking members for his welcome and describing the honour of addressing “this hallowed chamber”.
He told senators: “Quite simply, the United Kingdom will always be one of France’s closest allies and best friends.”
He also recalled how President Emmanuel Macron paid tribute to his mother, the late Queen, during her Platinum Jubilee last year and described her as the golden thread binding the nations together.
Charles told the Senate: “In the rich and complex tapestry of the relationship between France and the United Kingdom, my mother’s golden thread will forever shine brightly. Let it inspire us all to continue to weave the connections between our two countries – with determination, hope and with love.”
He thanked the People of France for the “great kindness you showed to us, and our people, at a time of such grief” after Elizabeth II’s death.
The King was welcomed by both the president of the senate and leader of the national assembly as well as a guard of honour.
He then signed visitors’ books before proceeding to the senate chamber, where his speech took place.
Charles was greeted with a round of applause when he quipped “no low blows and may the best win” between the French and English, Welsh and Scottish national teams competing in the Rugby World Cup being hosted in France.
Meanwhile, the Queen and Brigitte Macron, the wife of the French president, launched a new Franco-British literary prize at the Bibliotheque Nationale de France on the second day of the three day state visit.
Charles and Camilla later joined up to visit a community centre in Saint-Denis, northern Paris, where the Queen took on Mrs Macron at a game of table tennis.
The pair then went to a Paris flower market named after the late Queen.
As the rain poured during the afternoon, Charles and Camilla met two stallholders who met the King’s mother during her last state visit to France in 2014.
The royal couple were invited to sign the city’s official welcome book before making their way to Notre Dame cathedral.
They inspected a huge renovation project under way following the devastating fire which tore through the landmark in 2019, and met firefighters who tackled the blaze.
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