The Part-time Workers (Prevention of Less Favourable Treatment) Regulations state that a part-time worker is one who is "not identifiable as a full-time worker".
A full-time worker is a worker who is regarded, through custom and practice, in that establishment, as full-time.
In other words there is no specific formula. In some companies, workers who work 30 hours and above, might be customarily regarded as full-time, whereas in another company it might be only workers who work more than, say 35 hours a week.
To be considered as a part-time worker in your company, you have to be identified as someone who is not customarily regarded as full-time.
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