The track - co-written by Barlow and Lord Andrew Lloyd Webber for The Queen's Diamond Jubilee - has rapidly climbed to the top of the digital chart following tonight's documentary on BBC One.
Gary Barlow: On Her Majesty's Service aired earlier this evening (June 3), which showed the singer's journey throughout the Commonwealth as he recorded the song.
'Sing' was previously at the bottom end of the Top 10 before the documentary went out, but has now quickly risen to the number one spot.
Meanwhile, Barlow's solo version of the track has also climbed up into the iTunes top 20.
The single had entered the official singles chart at number 11 today, while the compilation LP went straight in at number one in the album chart.
Barlow has also curated the Diamond Jubilee concert, which will feature the likes of Robbie Williams, Jessie J, Sir Paul McCartney, Stevie Wonder and Sir Elton John performing at Buckingham Palace tomorrow (June 4).
Watch the video for 'Sing' below: