There is a quota on the number of singers who can go through to Bootcamp, and it appears the judges - Simon Cowell, Cheryl Fernandez Versini, Rita Ora and Nick Grimshaw - exceeded the limit.
A spokesperson for X Factor told The Mirror: "Owing to a very high standard of talent during this year's X Factor arena auditions, not all successful singers will be offered a place at Bootcamp.
"All singers have been made aware of this and are being told this weekend whether or not they will be going through to Bootcamp, after the judges have reviewed their auditions."
The Daily Star reported over the weekend that an email was sent to contestants notifying them of the procedure and warning that they may not make the cut.
The email is believed to have read: "The standard of auditions has been very high this year and, with a limited number of places available at Bootcamp, sadly we cannot offer everyone who received a 'yes' in their audition a place in the next stage of the competition.
"We are writing to inform you now as we want to be clear with you about the process and also to make sure that you do not book any travel for next week."
The next stage of the talent show starts today.
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