Is it unlawful to discriminate against part time workers?

Yes it is. Under the Part time Workers (Prevention of Less Favourable Treatment) Regulations workers have the statutory right, as part timers, to be treated the same as comparable full time workers, that is, workers on the same type of contract with the same employer. This right is not subject to any qualifying service requirements.

A part time work is entitled to receive, on request, a written statement of reasons for less favourable treatment from the employer. This statement may be taken into account by an employment tribunal.

The employer may be able to justify any difference in treatment, but only on objective grounds.