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 the normal full-time hours in that workplace. The rules apply to 'workers' which covers some consultants and contractors as well as employees.
Part-time workers have specific legal protections prohibiting less favourable treatment as compared to full-time workers. This impacts particularly on 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.