Royal expert on what started the feud between Harry, Meghan, William and Kate

The cause of the so-called royal feud between the Cambridges and the Sussexes is one of the biggest mysteries of the year.

Stories of alleged fighting between Prince William and Prince Harry have dominated much of the royal news, even threatening to overshadow major events including the birth of baby Archie.

Kate Middleton and Meghan Markle were dragged into the mess, with many pitting the sisters-in-laws against each other despite the duchesses showing no signs of tension during their joint engagements.

9Honey royal commentator Victoria Arbiter says that, although we'll never know for sure what sparked the feud, there are a few likely causes.

"There's been a lot of speculation that some of it started because Prince William, although he very much liked Meghan, and he loved seeing Harry happy with Meghan, he was concerned that perhaps things were moving at quite such a fast pace," Arbiter tells 9Honey.

Prince Harry and Meghan married in May 2018, just 16 months after they met.

"That's simply coming from the protective brother point of view," Arbiter explains.

"When they lost their mother, William stepped up even more to be the protective older brother, he's always looked out for Harry.

"And you have to remember both Chelsy Davey and Cressida Bonas said, 'Look, Harry, I'm crazy about you but I don't want any part of this circus,' so I think William was trying to be protective."

Meghan's background may have also played a role in the rift between the brothers.

"Meghan comes from another country, [so perhaps William said] 'does she know what she's getting in to?', 'do you know her well enough to know that she's up to this?', 'there's going to be tremendous pressure, have you both thought about this?'

"But of course, anyone who is in love, nobody wants their relationship questioned, so I think we also have two very stubborn brothers here and the chances are they were both bashing heads a little bit and things got a little contentious."

Arbiter believes Harry did himself no favours by refusing to deny rumours of a royal feud during his tell-all interview during the Africa royal tour.

"But what he did say was that so much of what has been talked about regarding a feud came from nothing," she says.

"And feud is such a big word – it really denotes big problems behind the scenes."

Recently, the Duke and Duchess of Sussex announced they would be skipping the traditional royal Christmas at Sandringham with the Queen, instead choosing to celebrate privately with Meghan's mother, Doria Ragland.

Many thought this was yet another sign of the ill-feeling between the Cambridges and Sussexes, rather than the usual balancing act all families struggle with during the festive season.

As to how the royal feud will end? Arbiter says 2020 is likely to herald in a fresh start.

"I think, at some point, one of them will be the bigger man, they'll both put their big boy trousers on and say, 'This is silly, blood is thicker than water, we need each other, we support each other, we love each other, we're both working to the greater good of the monarchy – let's move on'.

"Time is a great healer. Ultimately, these are two boys who love each other dearly and they'll always be in each other's corner."