In an address to the French Senate on Thursday, King Charles III underscored the importance of the special relationship between Britain and France, which he suggested could help them jointly tackle the climate emergency.
"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," Charles told French lawmakers.
"Quite simply, the United Kingdom will always be one of France's closest allies and best friends," he said, in a speech delivered almost entirely in French.
The king stressed that the two countries had a shared responsibility on issues from climate change to Ukraine.
"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," said the king, who has long been known for campaigning on environmental issues.
Charles suggested that France and Britain team up to tackle the climate and biodiversity emergencies with a new version of the Entente Cordiale, the 1904 pact that sealed the alliance between Paris and London.
"I would like to propose it also becomes an 'entente' for sustainability in order to tackle the global climate and biodiversity emergency more effectively," he said.
Later the same day, Charles and French President Emmanuel Macron attended a "climate mobilisation forum" at the Museum of Natural History in Paris that gathered companies and investors to discuss financing for poorer countries badly affected by climate change.
France and the UK are committed to "to increasing support for emerging and developing economies", they said in a joint statement.
The Elysee called climate and environmental concerns the "common thread" running through King Charles's visit.
"It's clearly a longstanding engagement for His Majesty and a conviction I hold deeply, on which we're working together," President Macron said.
Memories of Queen Elizabeth
The British monarch's three-day state visit is seen as a crucial opportunity to showcase the longstanding friendship between France and Britain, which both nations hope will help turn the page on years of tensions since the UK's exit from the European Union.
King Charles received a standing ovation following his speech to lawmakers, which was the first by a British monarch in the main chamber of the Senate and to representatives of both houses of the French parliament.
Remembering his mother Queen Elizabeth II, who passed away a little over one year ago, the monarch recalled that "my family and I were moved, beyond measure, by the tributes paid to her throughout France".
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Later on Thursday, King Charles and Queen Camilla visited a central Paris flower market named after the late monarch.
Then the royal couple toured the restoration works at Notre-Dame cathedral, which was ravaged by a fire in 2019.
They also visited the northern Paris suburb of Saint-Denis, home to the French national stadium used for the current Rugby World Cup and the Olympics next year.
After a formal farewell from Macron at the Elysee Palace, Charles and Camilla head to the south-west city of Bordeaux for the final leg of their trip on Friday.