As Tom Cruise is praised for holding Kate's hand, what are the rules on touching royals?

Tom Cruise has been called a "gentlemen" by social media users for holding the Duchess of Cambridge's hand while climbing the red carpet steps at the London premiere of Top Gun: Maverick on Thursday evening.

The pair had been walking the red carpet, alongside Prince William, when Cruise turned and offered his hand to Kate, before they walked up a set of stairs.

The duchess, who likely would have found it difficult to tackle the stairs unsupported in her inflexible Roland Mouret gown and Prada heels, accepted the gesture.

Read more: Duchess of Cambridge floors fans with 'Top Gun' dress: 'A supermodel and queen'

Social media users were quick to express their admiration for Cruise's gesture.

"He is always a gentleman and never forgets for a second how to behave in public," wrote one Twitter user.

"Tom Cruise being the perfect gentleman helping Kate with the stairs!!," said another.

A third tweeted, "Tom Cruise turns 'Prince Charming' for the gorgeous Duchess of Cambridge."

But others wondered whether Cruise, 59, was "allowed" to hold the duchess' hand.

It's unusual for non-royals - no matter how famous - to have physical contact with the Royal Family. But it's not unheard of.

"There are no official rules when it comes to touching a member of the Royal Family," says Omid Scobie, Yahoo UK's royal executive editor. "It's more down to what's acceptable in society or not.

"I think there are a lot of things that are perceived as royal etiquette or royal protocol that we assume might be the case but aren't actually."

Scobie adds that if the duchess had a problem with the gesture, then she wouldn't have given her hand to him.

"It could have been far worse," he says. "The worst would be for her to trip downstairs at a royal film premiere."

Scobie says the palace seems to put more emphasis on how to correctly address a member of the Royal Family.

The Royal website echoes this with: "There are no obligatory codes of behaviour when meeting The Queen or a member of the Royal Family, but many people wish to observe the traditional forms."

The ways to greet a member of the Royal Family are listed as:

• For men this is a neck bow (from the head only) whilst women do a small curtsy. Other people prefer simply to shake hands in the usual way

• On presentation to The Queen, the correct formal address is 'Your Majesty' and subsequently 'Ma'am,' pronounced with a short 'a,' as in 'jam'

• For male members of the Royal Family the same rules apply, with the title used in the first instance being 'Your Royal Highness' and subsequently 'Sir'

• For other female members of the Royal Family the first address is conventionally 'Your Royal Highness' and subsequently 'Ma'am'

And while these are also only suggestions based on tradition, Scobie again emphasises, "In terms of any rules about touching a member of the Royal Family, there isn't anything that they would instruct anyone."

Instead, it's about simply adhering to common sense.

"Yes, it's fine to give someone your arm or your hand if you're helping them up the stairs, but as we saw last night, no one rushed to run and hug William or Kate because that would be inappropriate," he says.

The lack of obligatory rules applies to the entire Royal Family.

"It's really sort of a one size fits all thing," says Scobie.

Read more: Royals on the red carpet: The best moments in pictures

"If you know the Queen well enough, you know that she prefers not to shake 100 hands. But it's not something that she shies away from either. I've never seen her refuse to shake someone's hand when presented with one."

In fact, it's quite the opposite. "Ultimately, I think people always forget that the Royals often go out of their way to make the people that they're talking to feel comfortable," he adds.

"I think they make a lot of effort as well to overlook a lot of this stuff."

Scobie references two past royal greetings that hit headlines for supposedly breaching etiquette: When LeBron James met the Duchess of Cambridge in New York in 2014 and put his arm around her for a picture and when Michelle Obama famously gave the Queen a hug in 2009.

Despite being unusual occurrences, neither have been "genuine breaches of protocol," Scobie explains.

Similarly, in 2013 during a visit from the Queen to West Quay fisheries at Newhaven Fish Market in East Sussex, it was reported the location guide touched Her Majesty's lower back - another supposed breach of etiquette.

"I remember at the time contacting the palace, and they basically said whether the woman had touched or not, the Queen wouldn't have taken offence either way, and there are no rules to suggest anything else," says Scobie.