Image copyright Getty Images Image caption Mackage's navy maxi coat on display in Nottingham - it sold out worldwide in hours

On the morning of 1 December last year staff at Canadian fashion house Mackage came into work to find a single item had inexplicably sold out overnight.

Every one of its £750 ($1,013) double-breasted military maxi coats had walked off the shelves in over twenty countries on three continents.

The reason? Meghan Markle.

Prince Harry's fiancée had worn the coat five hours earlier, five thousand miles away in her first public walkabout in Nottingham, England.

Within minutes the world wanted Mackage maxi coats.

Image copyright Getty Images Image caption Putting publicity in the right hands - Meghan Markle's Strathberry bag

Mackage were not the only winners. Meghan was also carrying a £500 ($675) midi tote bag made by Strathberry, a four-year-old start-up based in Edinburgh. The bag sold out in 11 minutes flat across its Global, US and China website.

Leeanne Hundleby, owner and co-founder of Strathberry, says: "Visitor numbers to the website were also amazing. At one stage they were up tenfold against our daily average."

Meghan wore a white coat by the Canadian brand Line the Label to announce her engagement with Harry, and which, yes, you've guessed it, sold out in minutes, reportedly crashing the firm's website.

Image copyright Getty Images Image caption Meghan in "The Meghan" at the royal engagement announcement

Line also stole a march on other brands caught up in the Meghan merchandising mania by christening the coat "The Meghan".

Royal gift

This is the kind of publicity that marketing departments dream of. Getting your design in front of Meghan could, if she wears it, splash it over every fashion website from Shanghai to L.A.

Ceril Campbell, a lifestyle and image consultant who styled and advised the Queen's granddaughter Zara Philips, manages the images of numerous A-List celebrities. She says: "When you move up seriously into the public eye on the A-List, you are inundated by people wanting you to wear their clothes. You literally have to fend them off."

Kensington Palace insists the Royal Family does not accept gifts, although Meghan's status, still officially a non-royal, is ambiguous in this respect. When the BBC asked Strathberry whether she had paid for her tote bag, it replied: "No comment."

Ms Campbell explained some of the thinking that goes into choosing the right designs. She says: "First you have to think what designer would be the most appropriate to the occasion, their nationality, their brand values.

Image copyright Getty Images Image caption Welsh jeans for a Welsh visit. Meghan and Harry in Cardiff

"Then there is the question of etiquette, whether you should say, have your shoulders covered... then there is the designer and where they come from; are they from the area you are visiting?

"Finally there is the price point - how much it costs. You don't want to be wearing designer outfits visiting people who are less fortunate - it's a question of being polite and considerate."

All of which explains why, say, visiting Cardiff, Meghan wore jeans by Hiut Denim - a Welsh brand dedicated to reviving the local denim manufacturing industry.

The Dress

And if you thought this is a bit over the top - it is nothing compared with what's coming up next: The Dress.

The moment Meghan steps from her carriage outside St George's Chapel in Windsor designers will be scribbling furiously to get a Meghan-inspired wedding dress onto paper and into shops.

"We are going to see Meghan's dress copied superfast", said Dr Patsy Perry, senior lecturer in fashion marketing at the University of Manchester. Fast fashion companies can get new designs in to shops in less than a month.

Image copyright Theia Image caption Not the royal wedding - one idea of what Meghan might look like

"You can get someone to do a pattern in a week, you can get it into sampling in another week if sample machinists are available. And then within a month easily it could be available to customers."

But why bother waiting? The wedding on the 19 May comes inconveniently late for the US bridal brand Theia's spring collection. So Don O'Neill, its creative director, gave it a royal theme and made an educated guess at what Meghan would be wearing - while confessing it was probably a bit too sexy for St George's Chapel.

Mr O'Neill says: "I think she'll be fully covered. She has to be. It's the Royal Family she's marrying into. But the dress will reveal the shape of her body. It'll be fitted to her. It'll have long sleeves, a high neck at the back, a little V in front, a seam running down the front covered with pearl buttons."

Brands boost

But it's not just a bridal dress that is being sold here. The impact of the real thing will go way beyond the the wedding aisle.

Image copyright Getty Images Image caption Kate Middleton's wedding dress - an impact on a whole industry

David Haigh, chief executive of the consultancy Brand Finance looked at how the Duchess of Cambridge's fashion choices had an impact on the industry.

He says: "Unofficial endorsements have proved a boon to British brands on numerous occasions in the past. Asda reported a 300% sales boost of its beige coats after the Duchess was seen wearing a similar looking Burberry design."

Mr Haigh believes the benefit to the fashion and apparel industry from Harry and Meghan's wedding will total around £150m this year. The wedding as a whole, when the effect of extra tourists, merchandising, champagne, wine and food sales is taken into account could add £1bn to the nation's economy.

Image copyright Getty Images Image caption Canadian fashion - Meghan's ear rings are from the Montreal jewellers Birks

But Meghan doesn't just buy British. She spent years living in her adopted city of Toronto where Suits, the cable TV legal drama in which she co-stared, was filmed, which explains her attachment to Canadian brands like Smythe, Line the Label, Birks the jewellers, and Sentaler.

Different dynamic

She also adds a different dynamic to Royal endorsements. She may not be shopping in Asda, but she is quite capable of sporting what the fashion industry euphemistically calls "affordable" designs.

At the Invictus Games (before her engagement) she wore a £300 patterned green dress from Self Portrait and a £100 maroon dress from Wilfred, one of the brands owned by the Canadian Aritzia chain.

Image copyright Getty Images Image caption Versatile - Meghan's £300 green dress from Self Portrait

Her versatility means she can unofficially endorse brands across a wide spectrum.

This is something she shares with Michelle Obama. David Yermack, professor of finance and business transformation at New York University's Stern School researched Mrs Obama's effect on the fashion industry. He estimated that the average value to a company following an appearance of an item worn by the First Lady was $14m.

"Obama mixes couture with items anyone can buy at a mall - she famously wore J Crew gloves while holding the Lincoln Bible at the Inauguration. Consumers flock to the stores, and even if they don't buy what she wears, they often leave with something else," he wrote.

Once you have felt the warm glow of the Meghan Markle effect on your sales figures, how long does it last?

Leeanne Hundleby of Strathberry says: "In terms of long term impact, it is difficult to say just now but we estimate sales this year will increase by up to 20-40% due to increased brand awareness."

Image copyright PA Image caption 1981: The dress the Emanuels made - and which, in turn, made them

David Haigh says: "Well, it could last a very long time indeed. Do you remember David and Elizabeth Emanuel who designed Diana's dress for her wedding in 1981? Everyone still knows them. Everyone remembers that dress and, even though they separated, it made their careers."