When Meghan Markle stepped out in London for her second official royal engagement on Tuesday, she broke traditional royal beauty protocol and tied her long brown hair up into a casual messy bun.

While the Duchess of Cambridge is never seen in public without a professional blow dry or an elaborate up-do, Markle decided to forgo all the fuss and rock the universal hair uniform of the busy modern woman, news.com.au reported.

Markle, 36, accompanied her new fiance Prince Harry during a visit to youth radio station Reprezent in Brixton. It was her second official royal duty following her successful visit to Nottingham in December.

The former Suits actress was praised by women on social media for her low-key, relatable look.

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Britain's Prince Harry and his fiancee Meghan Markle. Photo / AP
Britain's Prince Harry and his fiancee Meghan Markle. Photo / AP

Markle's entire outfit has already sold out online. She wore a coat by Canadian brand Smythe, chic $990 Burberry trousers, a Jigsaw scarf and a jumper from UK department store Marks & Spencer.

This is not the first time the "Meghan Effect" has taken off, with items the soon-to-be royal wearing selling out fast.

The £495 ($937) handbag she carried in Nottingham last year, from small Scottish label Strathberry, sold out in just minutes.

Prince Harry and his fiancee Meghan Markle. Photo / AP
Prince Harry and his fiancee Meghan Markle. Photo / AP

During her engagement announcement, Markle wore a belted white coat by Canadian brand Line The Label, which she wore over a £437 ($827) dark green dress by Parosh.

Line The Label's website crashed on that day, due to the number of people trying to access it.

Markle and Prince Harry announced their engagement late last year after months of speculation.

The couple will be married in a few months and live together in Kensington Palace.