Yes, it is. Under the Part-time Workers (Prevention of Less Favourable Treatment) Regulations, part-time workers have the statutory right to be treated the same as comparable full-time workers, i.e. workers on the same type of contract with the same employer. This is a right you enjoy from day one of your employment.
If you think you and your colleagues are receiving less favourable treatment because you are part-timers, you should request (and are entitled to receive), a written statement of reasons for the difference in treatment. This statement (or absence of it) may be taken into account by an employment tribunal hearing a complaint under the Regulations. The employer has to prove that this different treatment is justified on objective grounds.
For more information, see our Part-time Workers section.
In practice, it is usually best to deal with this sort of issue on a collective basis, rather than on your own. Join a trade union if you haven't done so already. Unions have many part-time workers and are expert at helping them to negotiate and to fight for better pay and conditions. Browse our Union Finder tool for more information.