Caroline Charles made the off-the-shoulder bugle-beaded emerald jacket and wide, bias-cut, double georgette trousers that Ms Thompson wore when she received the Best Actress award.
Ms Charles is a well-respected name in London. She dressed the Beatles in the Sixties; current customers include Selina Scott and Shakira Caine, wife of film star Michael Caine. But until last week her international impact had been small. The Thompson outfit will change that.
Ms Charles was trying hard to conceal her excitement last week at her Knightsbridge shop. She was failing miserably. 'The phone hasn't stopped ringing. I feel as though I won it. We've had American Vogue and W sending messengers over for clothes already.'
She was already reviewing plans for the US market. 'We may be looking at licensing deals.'
Media buyers say coverage of the Oscars is worth millions. Andy Tilley, director of strategic planning at Zenith Media, said: 'It's a brilliant opportunity.'
It is also a coup of staggering proportions. Italian and American designers spend heavily on persuading film stars to wear their frocks. They employ agents, fax sketches to nominees, and fly out fitters to keep the stars happy. Ms Charles spent nothing.
Last week, American designers consoled themselves by criticising the outfit, condemned on US television as 'a disaster'.
Italian designers had even more cause for sour grapes. Giorgio Armani dressed Jodie Foster, winner of the 1992 Oscar for Best Actress. This year, Valentino, another leading Italian designer, was hoping to dress Ms Thompson, as he had at the Golden Globes in January.
However, in February Ms Thompson decided to go British. Caroline Charles explained: 'She came here shopping several times, and then asked if I would do something special. She's got a lovely body, very curvy.'
Did the actress pay for her outfit? Ms Charles would say only that the dress was certainly not 'on loan', a practice common at the Oscars.
In Italy, Valentino's people are seething. When told the name of the designer Ms Thompson chose, the designer replied: 'Who's that?'