Yes, such discrimination is prohibited by the Part-time Workers (Prevention of Less Favourable Treatment) Regulations. Part-time workers have the statutory right, from day one of their job, to be treated the same as comparable full-time workers – that is, workers on the same type of contract with the same employer. The full-time worker you compare yourself with must be a real person who works where you work. It cannot be a hypothetical person, or an ex-colleague.
If you cut your own hours from full- to part-time but carry on doing the same job (for example when returning to work after maternity leave), the law allows you to compare your new part-time terms and conditions with the conditions you enjoyed when working full-time.
The employer has a defence if it can justify a difference between the terms and the full- and part-time workers’ terms on objective grounds.
A part-time worker is entitled to receive, on request, a written statement of reasons for less favourable treatment from the employer. This statement (or absence of it) may be taken into account by an employment tribunal hearing a complaint under the regulations.