A day after the eventing team won a first Olympic gold in 60 years, their counterparts in the dressage ended Germany's 40-year winning streak in the discipline.
It was a huge win - Britain had never even won a minor dressage medal before.
Carl Hester and Charlotte Dujardin posted Olympic record scores for their grand prix special tests to secure victory.
Britain finished the second phase of the competition on a total average score of 79.979 percent, well ahead of silver medallists Germany on 78.216 percent. The Netherlands took bronze.
Dujardin, who with horse Valegro achieved a world record score of 88.02 per cent for the grand prix special earlier this year, is now the proud holder of Olympic records for last week's grand prix test and Tuesday's grand prix special.
She scored 83.663 per cent for the grand prix and 83.286 per cent for the more technically demanding grand prix special, which is astonishing given the pair only started competing at the top ranks of international dressage last year.
It was also a major boost to Team GB, whose total of 20 gold medals outstrips the 19 won in Beijing, with more than five days of the Games remaining.
The last time Britain won 20 or more Olympic gold medals was at the 1908 Games, also in London.
Laura Trott added a cycling omnium gold to further extend Britain's total.