The track 'Morning Sun' is due for release on March 8, ahead of this year's Sport Relief weekend which takes place from March 19 to March 21.
Williams said of the news: "I am very happy to donate my time to help Sport Relief and I thank my co-writers for doing the same... Please join us in helping this very worthwhile cause."
All proceeds from the single will be donated to the charity.
The singer has also reportedly signed up for Simon Cowell's re-recording of 'Everybody Hurts', which is being released to raise funds following Haiti's recent earthquake disaster.