King Charles III joked about having "sausage fingers" with his son Prince William during rehearsals for the Coronation, a behind-the-scenes BBC documentary has captured.

The affectionate television portrait of the King reveals a playful and warm relationship with his son.

The documentary follows the build-up to the elaborate ceremony in May.

It also shows the Archbishop of Canterbury forgetting his lines in a rehearsal.

"I have a memory that is probably about as good as our spaniel - in other words zero," says Archbishop Justin Welby, about not knowing the words during one of numerous practice runs for the ceremony.

When the archbishop freezes mid-prayer, as he blesses the King in the Coronation chair, another clergyman teases: "You must have said this before."

The King, wearing the golden Coronation robes over his suit and tie, also dissolves into giggles.

The leisurely 90-minute documentary, Charles III: The Coronation Year, to be screened on BBC One on Boxing Day, shows King Charles as a good-natured figure, immersed in the complex preparations for his crowning.

A replica of part of Westminster Abbey was built inside Buckingham Palace, so the participants could keep practising.

In one of the final rehearsals, in the abbey itself, the cameras captured Prince William rather tenderly supporting his father.

When the prince struggles to fasten one of the ceremonial robes, the King tells him not to worry, as he does not have "sausage fingers" like his father.

There are scenes showing the intricate craft skills involved in preparations, including getting ready the historic crowns and the highly-decorated robes.

The documentary had private access to follow the first year of the new reign, after the death of Queen Elizabeth II.

The King is seen caught up in the emotions of the late Queen's funeral and there is an interview in which his sister Princess Anne speaks of the "serendipity" of being in Balmoral when her mother was dying.

The Princess Royal speaks of a strange "sense of relief" when she saw the crown taken off her mother's coffin, as though that symbolised the end of her long commitment to duty.

The film, written by royal author Robert Hardman, also sees the King showing his support for Ukrainian troops training for the war against Russia's invasion.

This is the biggest such television project of the new reign and it features glimpses of the King's daily working life, going through the red boxes of government documents, which we learn happens every day apart from Christmas Day and Easter Monday.

Prime Minister Rishi Sunak bobs about on a visit to meet the King, looking like an intern wondering how long he can stretch out the small talk.

There is a moment too that seems to appeal to the King's sense of humour, when he and his private secretary look at a message being sent to Sweden that is written in Swedish, when they realise they have no idea what they are sending.

It is a warm and sympathetic account of the new reign, with no glimpses of any difficult headlines from the year, whether about Prince Harry, Prince Andrew, a palace race row or protesters arrested at the Coronation.

But it shows the King and Queen as a couple strengthened by each other, starting a busy new stage in their lives when most people of their age would be ready to put up their feet.