There is no specific number of hours that makes one worker part-time and another full-time. A part-time worker legally just means any worker who works less than the normal full-time hours in that workplace. The rules apply to 'workers' which covers some consultants and contractors, as well as employees. For more information on the differences between workers, employees and consultants, read Consultants, workers and employees.
Part-time workers have specific legal protections prohibiting less favourable treatment as compared to full-time workers. This particularly impacts pay and benefits.
Part-time workers are also entitled to a written statement of reasons for any treatment that is less favourable than that of full-time workers. Where no justification for less favourable treatment is provided, part-time workers should first speak to their employer or trade union representative. They may then make a request in writing, to which their employer must respond within 21 days. Where the employer fails to provide a justifiable reason or the worker is not satisfied that the reason given was objectively justified, they may be able to bring a case to an Employment Tribunal.
Part-time workers are statistically more likely to be female than male. So, treating part-time workers less favourably may also be indirect sex discrimination. For more information, read Equal opportunities and discrimination.