The 22-year-old has been crafting her debut album with several high-profile US producers – including Mariah Carey’s long-term collaborator Walter Afanasieff - since she won the TV singing contest in December.
Cowell insisted that the LP will be worth the wait, saying: "We've spent ten months in a recording studio with Leona in LA and England. You're all here so I'll say it on the record: this album, in my opinion, is as good as early Whitney, early Mariah. I think it is a stunning record.
"We could have gone into the studio for a month, made the record quick, thrown it out. It would have been the wrong thing to do. We made a very public commitment to Leona."
Cowell's X Factor colleague Louis Walsh added: "It's like early Mariah Carey. I think Leona's the best female star from the UK in maybe 20 years."