Part-time workers have rights over and above the statutory rights applying to all workers.
These are contained in the Part-Time Workers (Prevention of Less Favourable Treatment) Regulations 2000 (PTWR). They are designed to reduce discrimination against those working part-time.
The regulations deem you a part-time worker if, quite simply, you are a worker whose hours are less than those of a full-time worker. As a part-time worker, you have the right not to receive less favourable treatment than a comparable full-time worker. You have the right to the same contractual benefits, and the right not to suffer any other detriment.
An employer is, however, able to treat part-time workers less favourably if they can demonstrate a justification on objective grounds for doing so. The conciliation service, ACAS, says an example of objective justification 'would include a part-time worker who is denied health insurance, even though a comparable full-time worker has one, because of the disproportionate cost to the organisation of providing the benefit' https://archive.acas.org.uk/index.aspx?articleid=1576 .
The regulations say you have the right to be considered no less favourably than an equivalent, full-time 'comparator', against whom you can compare your own treatment. In practice, you need to be able to compare your situation to that of a real-life person, not a hypothetical one. The comparator needs to be employed by the same employer, under the same sort of contract, doing the same or broadly similar work. Additionally, the comparator needs to be employed at the same time as the part-time worker. You cannot compare your treatment to that of someone who no longer works where you do.
In the workplace, this means that if you go from full-time to part-time, without breaking the continuity of your employment, you can compare your part-time terms and conditions with those which applied before your hours were cut. This situation would apply, for example, to women who reduce their hours when returning to work from maternity leave.
If you are a part-time worker, and you believe you have been treated less favourably because of that, you have the right to request a 'written statement of reasons for any treatment which is less favourable than a comparable full time worker'.
- More information from ACAS: Part time workers.