Firth appeared to dive into a lake and swim underwater before climbing out with his thin white cotton shirt clinging to his torso for the BBC's 1995 Pride and Prejudice mini series.
It was the scene which made him a household name and propelled him to fame as a film actor.
But now the programme's director has disclosed that Firth never actually entered the lake.
Health and safety concerns meant a stuntman took the plunge instead because there were worries that Firth might catch a waterborne disease.
The footage of him underwater was filmed in a tank at Ealing Studios in West London, far from Lyme Park in Cheshire where the rest of the scene was shot.
Director Simon Langton said there were difficulties with the actor's insurance and special arrangements had to be made so that Firth didn't enter the lake.
He said: "It was a stuntman.
"We didn't want our leading man to catch Weil's disease, which can be caught from rat urine in water."
It was one of many problems the crew encountered when setting up the moment in which Mr Darcy takes an impromptu swim before meeting Elizabeth Bennet, the object of his unfulfilled desire.
It was originally intended that Firth should strip off and leap into the water naked.
But this had to be changed because the BBC felt nudity would be too much for a Jane Austen costume drama.
Then producers considered having Firth dive in wearing his underwear.
But that was decided this would be historically incorrect.
In the end they opted to have him leap in fully clothed, bar his waistcoat and overcoat.
But fans are assured the footage of Mr Darcy striding out of the water in his drenched clothing was all shot using Firth.
Mr Langton told the Daily Mail: "Nobody had the slightest inkling that Colin Firth, wearing a lightweight cotton voile shirt with his nipples showing underneath, would have such an effect."
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