According to The Sun, the 25-year-old singer - who announced that she was leaving the music industry earlier this year - will pen 12 songs for the show.
"It's a huge deal and a huge job. Lily's not on a roll yet as she's only finished one but she has a lot of ideas," a source claimed.
Allen recently confirmed that she is planning a new career in the fashion industry but will also embark on other projects.
