The track will be officially launched on Monday, the star confirmed.
"'Challa' will be out on the 1st of Oct. Looking forward to it... haven't seen it since we finished shooting it," Khan wrote on Twitter.
Khan will be seen in the role of a busker playing the guitar at locations across London, including Trafalgar Square, Butlers Wharf, the South Bank, Canary Wharf and St Pancras Station.
The Rabbi Shegill-performed song has been composed by AR Rahman, with lyrics from Gulzar.
Khan and director Yash Chopra recently sang a line from the song at an event to celebrate the filmmaker's 80th birthday.
The Yash Raj Films production releases on November 13.
Watch the Jab Tak Hai Jaan promo below: