British head of state King Charles III's upcoming visit to France will underscore historic close relations between the two countries, London and Paris said.

Ties between the two neighbours have been strained in recent years, largely over the UK's exit from the European Union but also migrants crossing the Channel from France in small boats.

Charles's state visit to Paris and Bordeaux on September 20-22 is being seen as part of a wider reset between the G7 and NATO allies that began under Prime Minister Rishi Sunak.

"The State Visit will celebrate Britain's relationship with France, marking our shared histories, culture and values," deputy private secretary to the king, Chris Fitzgerald, told reporters.

"It will also provide an opportunity to look forwards and demonstrate the many ways the UK and France are working together."

A senior UK foreign office official said relations had been "steadily improving" since Sunak and his senior ministers travelled to Paris in March for a bilateral summit with their French counterparts.

Sunak's relationship with Macron -- who also has a background in finance -- is markedly different from his controversial predecessors Boris Johnson and Liz Truss.

At one point, Truss even said "the jury's out" on whether the French leader was a "friend or foe" of the UK.

In Paris, Macron's office called the visit "an honour for France and illustrates the depth of the historic ties that unite our two countries".

The Elysee also played up the personal relationship between Macron and Charles, even before he became king last September.

The visit "symbolises the relationship of friendship and trust between the President of the Republic and His Majesty the King", it added.

Macron, whose tribute to Charles's mother Queen Elizabeth II after her death was well-received in the UK, made an open invitation to the new monarch to visit France at her funeral.

- Rescheduled -

The trip had been due to go ahead in March but was postponed by widespread civil unrest in France due to Macron's unpopular pension reforms.

Buckingham Palace was keeping tight-lipped on exact details of the visit but officials are hoping it will be as successful as his trip to Germany.

That trip, originally intended to follow the France state visit, went ahead as planned to Berlin and Hamburg.

The foreign office official said it had already led to "very real benefits" for the UK.

Highlights of the Paris leg include a wreath-laying ceremony at the Arc de Triomphe and a banquet at the Palace of Versailles, west of the capital.

Charles, 74, will also address senators and members of the National Assembly at the French Senate, Fitzgerald said.

The king and his wife Queen Camilla, 76, will meet community groups and sports star as France hosts the Rugby World Cup and Olympics.

The two heads of state will also discuss climate change, carrying on from their talks at the COP26 event in 2021 and other recent initiatives.

In Bordeaux, Charles -- a lifelong environmentalist whose Aston Martin car runs on biofuel made from surplus English white wine and whey from cheese manufacturing -- will tour an organic vineyard.

He will also meet firefighters involved in tackling last year's wildfires in southwest France.

Charles, a fluent French speaker like his mother, made 34 trips to France as Prince of Wales between 1969 and 2022.

He was last in France in June 2019 to attend a memorial service for the 75th anniversary of the D-Day landings in World War II.