At an after-party for the premiere of the King’s environmental documentary at Windsor Castle, one comment was heard above the chatter: “It’s always awkward watching yourself. I can’t imagine ever getting used to it.”

They were not Charles’s words, but Benedict Cumberbatch’s. The actor was standing in the Grand Reception Room with other celebrities who had come out in support of the King.

The Duchess of Edinburgh speaks to Jools Holland, Benedict Cumberbatch and Sophie Hunter

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Although Cumberbatch’s fanbase is legendary — his devotees have been known to camp outside box offices to get tickets — it seems that the new court of King Charles has attracted its own fans, and many of them are rather well known.

On Wednesday night in the Waterloo Chamber at Windsor Castle, where Queen Victoria once invited actors to perform Sharkespeare, the court of the new Carolean order were now invited to watch Finding Harmony: A King’s Vision, which will be released on Amazon Prime next week. It is thought to be the first film premiere hosted in a British royal residence.

Dame Judi Dench and Sir Kenneth Branagh joined Cumberbatch and others to watch the documentary.

Charles with Stanley Tucci and Dame Judi Dench

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Sir Kenneth Branagh and Kate Winslet

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Kate Winslet, who played Ophelia in Branagh’s 1996 film adaptation of Hamlet, has narrated the King’s film and described Charles’s campaigning as “inspiring”.

In the 90-minute documentary, Charles borrowed a line from Hamlet by saying that he hoped to make a difference to the planet “before I shuffle off this mortal coil”.

Afterwards, Cumberbatch empathised with the King about watching oneself in technicolour on the big screen as Winslet gripped Branagh round the waist before blowing him kisses as she moved away.

Also in attendance were Stanley Tucci, Fearne Cotton, Sir Rod Stewart and his wife Penny Lancaster, Jools Holland, the broadcaster Sarah Beeny, the astronaut Tim Peake and the chef Raymond Blanc. Sir David Beckham and Meryl Streep, who have supported the project, were abroad and sent their regrets.

Camilla and Dench in 2018. They have been friends for many years

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The event was something of a departure from the formality of the previous reign and the dignitaries who would hitherto have filled the chairs at Windsor Castle.

A royal source said: “Monarchy has always had a certain type of magnetism but we’re certainly seeing a more informal style of ‘social court’ in that sense. We call it ‘formal informality’. The King is not throwing the baby out with the royal bath water when it comes to tradition and ceremony but equally he thrives on connecting with people from all backgrounds on a human level and the Harmony film shows that.”

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Celebrity engagement with the film has been boosted by the force of the King’s personality and by the fact that his views on the environment and organic farming, once derided, are now on trend.

A friend of the King noted that in a world where the “authentic self” won acclaim on social media, Charles had always been someone who has stood for what he believed in and — as he says in his documentary — he “wasn’t going to be diverted” from his green campaign.

Penny Lancaster and Sir Rod Stewart at the premiere

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His wife has also played a large part in attracting celebrities to the court of King Charles. Dench, a friend of Camilla’s, came to the couple’s defence weeks into the new reign when she wrote a letter to The Times outlining the many injustices levelled at the royal family by The Crown, the Netflix series in which Charles was seen plotting to force his mother to abdicate.

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She described the series as “both cruelly unjust to the individuals and damaging to the institution they represent”, and called for Netflix to add a disclaimer telling viewers that it was a “fictionalised drama”.

The celebrity bubble around the King is expected to grow even larger when the King’s Trust, formerly the Prince’s Trust, celebrates its 50th anniversary. The milestone will be supported by Sir Mick Jagger, Sir Elton John, Sir Idris Elba, Claudia Winkleman, Sir Gareth Southgate and Damian Lewis.

Asked about the King’s celebrity stardust, courtiers were unfazed by the bright lights of Hollywood.

“The King is a man who thrives in company, not just from celebrities but people from all walks of life — he feeds off their knowledge, their humour and passions across a huge diversity of sectors,” a Palace source said. “He’s certainly not interested in celebrity for fame’s sake, but rather in expertise and enthusiasm.”

Kristina Murrin, chief executive of the King’s Foundation, which produced the new documentary with Amazon, said: “The star-studded line-up at the Finding Harmony premiere witnessed the King in his element — as an advocate for nature.

“It’s an exciting time for those who can remember when the King was ridiculed for these ideas that have now been proved to be so timely and perceptive.”