The Westcountry author of the children’s book War Horse led the birthday well wishes for the Duchess of Cambridge, while heaping praise on the director who turned his novel into a Oscar-tipped film.

Michael Morpurgo, from Iddesleigh in Devon, was last night at the film’s premiere in London, where he was joined by the royals, and director Steven Spielberg.

Alongside the Duke of Cambridge, Kate, who turns 30 today, graced the red carpet wearing a floor-length black lace gown by Somerset-born designer Alice Temperley.

The long-sleeved dress, which was nipped in at the waist, was accessorised with a black clutch bag and intricate diamond jewellery.

The couple shared the red carpet with the star of the film – a thoroughbred called Joey, who was driven from his stables in Milton Keynes to face fans and photographers.

Mr Morpurgo said it was an “honour” that the Duchess chose War Horse, which was partly filmed on Dartmoor, as her first royal premiere and said he would send her a copy of the book as a birthday gift.

“I am thrilled, it is a wonderful honour that she has chosen this film to do that,” he said.

“I will send her a copy of War Horse for her birthday.

“She is a wonderful example of dignity and calm and I like that in her.”

The premiere of Steven Spielberg’s film, in London’s Leicester Square, was also attended by 600 serving and ex-serving military personnel and their families, as well as beneficiaries of military charities. The royal couple went to the event in aid of the Foundation of Prince William and Prince Harry.

The royal couple met military personal as well as the cast and crew. They also chatted animatedly with Sherlock actor Benedict Cumberbatch, and Emily Watson, who both star in War Horse.

Actor Jeremy Irvine, from Cambridgeshire, said that the couple were “delightful” when he met them, adding: “I am a little bit overwhelmed. It feels like a long way from home.”

He joked that on the set of the film there were more make-up artists for horses than there were for people after seeing an assistant scoop up the mess the horse left behind on the red carpet.

The First World War epic, which was nominated for best drama at the Golden Globes, is based on Mr Morpurgo’s novel.

It follows on from the success of the play, which debuted at the National Theatre in London in 2007 before sell-out West End and Broadway runs.

Paying tribute to the several-times award-winning Mr Spielberg, the novelist said: “Any man who can tell ET and Schindler’s List in the same lifetime is an extraordinary person.”

War Horse opens in cinemas on Friday.