PARIS - King Charles III on Thursday pledged to do everything he could to strengthen the relationship between France and Britain, suggesting the 'indispensable' partners could also team up to jointly tackle the climate emergency.

In a speech at the French Senate on the second day of a three-day visit to France that London hopes will tighten post-Brexit relations, Charles deftly mixed English and French, winning a standing ovation from the lawmakers.

He recalled his mother Queen Elizabeth II, whom he succeeded upon her death one year ago, describing her legacy for France-UK relations as a 'golden thread which will forever shine brightly' and saying the royal family were 'moved beyond measure' by tributes to her from France.

'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,' he said.

'Quite simply, the United Kingdom will always be one of France's closest allies and best friends,' he said, speaking from a lectern adorned with British, French and EU flags.

He suggested that France and Britain should team up to tackle the climate and biodiversity emergencies with a new version of the 1904 Entente Cordiale pact, which sealed the friendship between Paris and London.

'I would like to propose it also becomes an 'Entente pour la Durabilite' (Partnership for Sustainability) in order to tackle the global climate and biodiversity emergency more effectively.'

He also vowed that London and Paris were 'steadfast in our determination Ukraine will triumph' in fighting the Russian invasion.

'Our alliance and our resolve are as important as ever. Together, we stand in resolute solidarity with the Ukrainian people.'

His speech will also be etched in history as the first British monarch to speak in the main chamber of the Senate: his mother spoke in a conference hall within the Senate in a 2004 speech.

French President Emmanuel Macron, right, and Britain's King Charles III toast during a state dinner in the Hall of Mirrors at the Chateau de Versailles in Versailles, west of Paris, Sept. 20, 2023.

Packed schedule

Charles's speech at France's upper house of parliament is the diplomatic high point of the day that followed Wednesday's banquet at the Versailles Palace hosted by President Emmanuel Macron.

He will later visit 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 -- where he is expected to see residents and sports stars.

Also heading to the Ile de la Cite on the river Seine, Charles -- a keen gardener who once admitted he talked to his plants -- will tour a flower market named after Queen Elizabeth II on her last state visit in 2014.

From there, he will view renovation and reconstruction work at the nearby Notre-Dame Cathedral, which was partially destroyed by a devastating fire in 2019.

Charles had said in an emotional message to Macron after the fire that he was 'utterly heartbroken', calling Notre-Dame 'one of the greatest architectural achievements of Western civilization'.

The Paris leg of the state visit wraps up with a formal farewell from Macron at the Elysee Palace.

The visit, which was rescheduled from March because of mass protests against French pension reforms, also aims to showcase Charles's stature as a statesman just over a year after his mother's death.

The original itinerary in Paris and the southwestern city of Bordeaux is largely unchanged, and is packed with ceremony and pomp in a country that abolished its monarchy in the 1789 revolution and executed its king.

The French president is known to have a strong personal rapport with Charles, with both men known for their love of books.

Commentators in France excitedly noted how Macron had repeatedly touched Charles's shoulder and Brigitte Macron kissed Camilla, in a new protocol unthinkable under the more distant and austere Elizabeth II.

Macron presented Charles with a book by the 20th-century French writer Romain Gary, while he received a special edition of Voltaire's 'Lettres sur les Anglais' ('Letters on the English').