The father-of-three is said to be recording songs for the forthcoming children's film produced by Dreamworks.
As part of the plot, Pat is thought to enter a talent show fronted by "an oily, smarmy guy" called Simon Cowbell. According to the paper, Barlow will provide a selection of post-themed songs that the character performs during the fictional programme.
Postman Pat: The Movie - You Know You're The One will also star Jim Broadbent, David Tennant, Rupert Grint and Kate Winslet, who has been cast as Pat's wife.
Green Wing star Stephen Mangan is providing the speaking voice of Postman Pat in the film.
"I'm absolutely thrilled to be playing the world's best-known postman in his big-screen debut," he said. "He's an iconic British hero and it's a great script.
"There are songs like 'Wait A Minute Mr Postman' and 'Signed, Sealed Delivered'. I was so excited when I got that job. I said to my 5-year-old son Harry: 'I'm Postman Pat' and he said: 'No you're not.' Still, therapy will sort that out!"
Postman Pat: The Movie - You Know You're The One is due to be released in 2013.