Can an employer treat part-time workers less favourably than it treats full-time workers?

Not unless such treatment can be objectively justified. The Part-time Workers (Prevention of Less Favourable Treatment) Regulations 2000 (SI 2000/1551), which implement the Part-time Workers Directive (97/81/EC), state that part-time workers must not be treated less favourably than comparable full-time workers in matters relating to pay, overtime premium payments, holiday and holiday pay, sickness benefits, selection for redundancy, access to pension schemes and other contractual benefits.