LONDON (AFP) - The gesture is so mechanical that few people even notice it.

Yet, when Ms Meghan Markle, the American actress turned newlywed wife of Britain's Prince Harry, closed her own car door, a wave of shock and awe rolled over Britain.

Videos of the brazen moment flooded social media and generated headlines that searched for adjectives to describe the momentous nature of what had just occurred.

"Meghan Markle leaves people in shock as she shuts own CAR DOOR on first solo engagement," the Express reported.

It included the footage of Ms Markle, 37, stepping out of a luxurious black sedan and being greeted on her arrival on Tuesday at her first solo engagement - the opening of an exhibition at the Royal Academy of Arts in London.

The man in a tuxedo, who welcomed her with a warm handshake, appeared to be preparing to close the car door. However, he quickly swiped his hand over his mouth the moment Ms Markle, now the Duchess of Sussex, closed the door behind her without looking.

The BBC summarised the mood of the nation in its own headline: "Meghan closes a car door and some people can't handle it."

Video: the Duchess of Sussex in Givenchy tonight @royalacademy pic.twitter.com/zsRzIZi6kV — Rebecca English (@RE_DailyMail) September 25, 2018

Ms Markle's every smile, step and wave has been scrutinised by royalty watchers curious to see how the mixed-race American actress will perform in her new role.

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She is adored by the public and shadowed at every appearance by the press.

Many saw it as a sign that Ms Markle remained a down-to-earth woman without pretences, despite being married to the grandson of Queen Elizabeth II.

"A princess who still takes the time to shut her car door," Ms Emily Andrews, the royal correspondent for The Sun tabloid, tweeted. "Well done Meghan!"

But one royal correspondent tried to bring everyone back to their senses by noting that Ms Markle had not, in fact, breached any protocol.

"Usually royalty and dignitaries have someone to open/close car doors for security reasons: Nothing to do with 'princess behaviour'," tweeted Mr William Hanson, an etiquette coach and columnist for the Daily Mail's website.

"Meghan clearly just shut the door out of force of habit."