The track - a joint project between the pair to commemorate the 60th anniversary of Queen Elizabeth II's coronation this summer - also features Gareth Malone, The Military Wives and The Commonwealth Band.
The song received its first play on BBC Radio 2's Chris Evans show this morning (May 18).
The anthemic song features 230 people and musicians from every Commonwealth state, including the Queen's grandson Prince Harry performing a tambourine solo.
Speaking about writing the track with Barlow, Lord Lloyd Webber said: "We got together with embryonic ideas. Gary had an idea for the chorus, I had the idea for the verse, we both agreed that the message was about the Commonwealth.
> Gary Barlow posts pictures from Commonwealth travels
"I was thinking 'anthem', but we had to involve all the people we could, and it had to be something very simple that people could learn. Working with Gary has been one of the great joys, he is such a fantastic songwriter, it was a completely new experience."
Barlow added: "I'm very happy, it sounds good. There's 230 people on there, from thousands of miles around three continents. I don't know of a record that's been made like this before, I've certainly had a lot of fun making it."
On what the Queen thought about the track, he said: "She was most interested in the stories of the people we had on there, as it represents her Commonwealth, the thing she's the most proud of. She was tapping along."
> Alan Titchmarsh presents Queen Elizabeth II documentary for ITV
'Sing' will be available to purchase from May 28, while sheet music can be downloaded for free from today.
The video can be streamed online from 9am today, while it will receive its TV premiere on BBC One's The One Show tonight at 7pm.
Gary Barlow will front a documentary about the making of the track on BBC One on June 3.
The X Factor judge has also been curating The Queen's Diamond Jubilee Concert, to be held on June 4.
Robbie Williams, Jessie J, Sir Paul McCartney, Stevie Wonder and Sir Elton John are among the long list of artists who will perform at Buckingham Palace.