Right Royal Relief: The look that says Phew, I'm glad that's over! Moment the King returned to Buckingham Palace with Queen Camilla after his Coronation is captured in landmark documentary

When the curtain falls after a theatrical showstopper, actors share applause and pleasantries as the buzz of adrenaline is replaced with a warm rush of relief.

It was much the same for King Charles after his Coronation – with the minor addition of receiving a thunderous Royal salute from 4,000 soldiers.

A landmark documentary has captured the moment the King returned to Buckingham Palace with Queen Camilla after the service at Westminster Abbey on May 6.

His sister, the Princess Royal, likens the poignant moment to that of an actor who has performed to the best of their ability, then exited the stage to hefty applause.

She tells the makers of Charles III: The Coronation Year, which is due to be broadcast on Boxing Day: 'Ask any actor who comes off stage having done a performance that they really put a lot into. It's that kind of relief.'

The 90-minute film is written and co-produced for Oxford Films by the Mail's Royal expert, Robert Hardman, and is a highlight of the BBC's Christmas schedule.

It is the first time a crew has been given fly-on-the-wall access to the Royal Family in the run-up to such a momentous national event.

It is written and co-produced for Oxford Films by the Mail's own royal expert, Robert Hardman, who gives more delightful detail in today's Weekend Magazine.

The programme includes interviews with family members, notably Princess Anne, and staff responsible for making the King's big day such a success.

On the morning of Saturday, May 6, King Charles and Queen Camilla are involved in last minute preparations at Buckingham Palace. 

Along the corridor, Her Majesty walks out in her Bruce Oldfield embroidered ivory gown, accompanied by her pages – who she affectionately calls 'the lads'.

'Very slow, together,' she says. Lady Lansdowne, one of Camilla's two official ladies in attendance on the day, says of the moment: 'It wasn't until we were all ready that we actually all came together to see each other for the first time.

'She hadn't seen our dresses and we hadn't seen her dress. That was a very special moment. It was quite like a wedding. It was the bridesmaids going to see the bride.'

The footage shows the Queen gingerly walking towards the exit, accompanied by her three-grandsons and great-nephew.

'Don't tread on my dress or that's going to a be a problem,' Camilla affectionately reminds them.

She adds to the cameraman, with a smile: 'Here we are, with all the lads.' 

The King then appears talking to his equerry Lieutenant Colonel Johnny Thompson, smilingly helping to diffuse everyone's nervousness by flapping the Robe of State worn by his grandfather, King George VI, at his Coronation in 1937, in his own 'I can fly' Titanic moment.

Camilla also jokes: 'Someone always gets pulled over,' referring to the weight of their finery.

As the couple depart in the Diamond Jubilee State Coach for Westminster Abbey, Lady Lansdowne, lifelong friend of the Queen, remarks: 'There was just a really exciting moment of getting them into that carriage for the first time and then we were off.'

In an exceptionally moving segment, Camilla's other lady in attendance, her sister Annabel Elliot, waves her off and dabs at her face with a handkerchief, apparently moved to tears. 

She recalls: 'I thought back to being two years old and watching the Queen's [Elizabeth] Coronation on a tiny black and white television – and there goes this golden coach with my sister in it.

'It's so surreal and this cannot be happening. Yes, it was quite a moment.' Lady Lansdowne adds: 'We were ready to go out and face literally the world.'