'You knew that was coming!': Hilarious moment King Charles comes to the rescue when Archbishop of Canterbury Justin Welby forgets his lines in Coronation rehearsal

Practicing for what would be the biggest moment in his time as a member of the clergy, Justin Welby can be forgiven for a case of nerves.

And new footage of rehearsals for King Charles III's coronation, released by the BBC ahead of being broadcast in full on Boxing Day, reveals that not even the Archbishop of Canterbury is immune to fluffing his lines.

As Charles sits on the throne at what would be quite literally the crowning moment of his life, the Archbishop struggles to remember the final words of what should be a straightforward blessing.

Mr Welby says: '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 leans over and helpfully adds, 'and always', joking: 'You must have said this before?'

The off-the-cuff remark prompts laughter from everyone present - including the King. 

A grinning Charles, shaking his head with laughter, then adds: 'You knew that was coming.' 

Mr Welby then finishes the blessing, cheerily surmising: 'Be with you and remain with you always. Amen.'

The archbishop told the documentary Charles III: The Coronation Year: 'I have a memory that is probably about as good as our spaniel's — in other words, zero.

'He (Charles) gave me a nice smile and nodded his head. But that was a glitch.'

The scene is one of a number of previously unseen pieces of footage being broadcast for the first time in the Boxing Day documentary, which is narrated by Helena Bonham Carter.

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 - and is a cornerstone of the BBC's Christmas schedule.

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.

An unlikely candid moment sees the King, amid last-minute preparations at Buckingham Palace, flap the velvet and ermine Robe of State and proclaim excitedly: 'I can fly!'

The footage, captured on the morning of Saturday May 6, saw Camilla walk out in her Bruce Oldfield embroidered ivory gown, accompanied by her pages – who she affectionately calls 'the lads'.

'Very slow, together,' she says.

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

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.'

'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.'

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.