The hitmaker's Happy has become the most downloaded track ever in the U.K.
Thicke's Blurred Lines, which Williams produced, was the previous top download.
Pharrell has three songs in the all-time top 10 - Happy, Blurred Lines and Get Lucky, his collaboration with Daft Punk. All three songs have sold more than one million copies within a year, a feat only matched by the Beatles in Britain.
Upon learning the news, Williams told OfficialCharts.com, "I would just like to say thank you, first and foremost, to the people, because the charts are determined by them. I have a lot to be thankful for because they've chosen these things and it's resulted in Happy being what it's become."