The Prince of Wales avoids playing cards with the “ruthless” Middleton family, his brother-in-law James Middleton has revealed.

The heir to the throne has spent a lot of time with his in-laws over the years, after meeting his now-wife Catherine, Princess of Wales, at university.

Now his brother-in-law has revealed in a poignant new book that Prince William always tries to get out of family card games because the Middleton clan is always so determined “to win at all costs”.

“William would flinch at our ruthless determination to win at all costs. He’d be delighted to be the first out, and when no longer compelled to take part, he’d slink off to cuddle [my dog] Ella,” Middleton writes in his book Meet Ella: The Dog Who Saved My Life.

“Better still, he’d absent himself from the game entirely. ‘James, does Ella need a walk?’ he’d ask, before we’d even started dealing the cards.

“My sisters and I would exchange a knowing glance. William, for all the competitive rigour of his military training, was happy to be a loser at cards.”

The anecdote is one of several personal revelations about the relationship between William and Catherine in the book, which has been serialised in the Daily Mail before its publication next week.

It follows the release earlier in September of an astonishingly personal video by the Princess of Wales, in which she revealed she had finally finished chemotherapy for the cancer discovered earlier this year.

The three-minute video features the Wales family at home at Anmer Hall in Norfolk. Catherine’s parents, Michael and Carole Middleton, were also prominent – including scenes where all are playing cards.

William and Kate, with their two oldest children, head to a Christmas church service, accompanied by the Middletons. Photo: Getty

Middleton also opens up about his mental health struggles and credits his spaniel Ella with helping him through some of his darkest moments.

He reveals he pushed away his famously close family – including sisters Catherine and Pippa Matthews – when he was at his lowest.

“I contemplate ways of dying so I can get off the giddy roller-coaster that is sending me to the brink of madness. I cannot sleep because my mind is in tumult,” he writes.

“The insomnia is dizzying. I am utterly exhausted. I feel misunderstood; a complete failure. I wouldn’t wish the sense of worthlessness and desperation, the isolation and loneliness, on my worst enemy. I think I’m going crazy.

“Yet I know I am privileged; fortunate, too, to have a loving and close-knit family – mum and dad, my sisters Catherine and Pippa, their husbands William and James – but I push them all away.

“I do not answer their phone calls. Emails remain ignored. Invitations to visit go unheeded. I hide behind a double-locked door, unreachable.”

He then recalled that it was his beloved black spaniel who would save him time and time again.

“I haul myself back from the brink, slowly climb down the ladder and stroke Ella’s silky head. She is the reason I do not take that fatal leap. She is Ella, the dog who saved my life,” he writes.

Middleton also shares a story about time spent with the late Queen Elizabeth at Sandringham at Christmas.

“One year, the queen and I sat down to do a jigsaw puzzle. It was the sort of activity I’d have enjoyed with my own grandparents, all four of whom had died in the space of three years when I was a teenager. So in a way, I felt the queen was filling a granny-sized void in my life,” he writes.

“There we were, engaged in this everyday pleasure, which was elevated to the extraordinary by the company I was in. It still feels surreal, the fact that I was there with the queen: I look back on it with amazement.”

Middleton is now married to Alizée Thevenet, and the couple have a one-year-old son.

-with AAP