Landmark royal documentary narrated by Helena Bonham Carter takes behind the scenes look at King Charles's first year as monarch... including the moment before his Coronation when he donned his ermine gown and joked 'I can fly!'

It is a moment of wide-eyed, almost childish excitement amid the making of history.

King Charles, moments before leaving Buckingham Palace for his Coronation in May, flaps his velvet and ermine Robe of State and laughs: 'I can fly!'

The candid moment is captured in a landmark new 90-minute documentary, narrated by Helena Bonham Carter, which offers an exclusive behind the scenes look at his first year as monarch.

Charles III: The Coronation Year, to be broadcast on Boxing Day, 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.' 

Other charming footage shows a rehearsal at the Abbey. Charles and Prince William, are practising with the Archbishop of Canterbury, Justin Welby.

The Archbishop says later on camera his memory is worse than his family dog's and he gets his words muddled up, much to the King's hilarity.

He says: 'I have a memory that is probably about as good as our spaniel's – in other words, zero.'

He rattles off what should be a familiar blessing: 'And the blessing of God Almighty, the Father, the Son and the Holy Spirit, be amongst you and remain with you now and... no that can't be right?' 

The Archbishop of York interjects 'and always' helpfully, adding jokingly: 'You must have said this before?' Everyone – including the King – is helpless with laughter.

'You knew that was coming,' the monarch chuckles.

Welby continues: 'Be with you and remain with you always.' Still smiling and shaking his head, Charles adds: 'Amen. Marvellous.'

Princess Anne emphasises her brother's devotion to duty.

'Monarchy is a 365 days a year occupation. And it doesn't stop because you change monarchs for whatever reason,' she says.

Charles III: The Coronation Year will air on Boxing Day at 6.50pm on BBC One.