The Grammy winners, who previously worked together on hits like Hollaback Girl and Stefani's new single Spark the Fire, have co-written Shine for the family friendly film.
Williams has a good track record for composing tracks for such movies - his original Despicable Me 2 song Happy won him an Oscar nomination and became one of the last year's biggest hits.
He says, "What a wonderful opportunity, as a parent, to contribute to something as classic, authentic and generational to all of our lives, as Paddington Bear."
The film adaptation of author Michael Bond's beloved classic children's books will hit cinema screens in Britain this Christmas (14). The movie will be released in the U.S. early in 2015.