The Part-Time Workers (Prevention of Less Favourable Treatment) Regulations 2000 (the “PTWR”) gives part-time workers the right not to be treated less favourably by their employer than a comparable full-time worker. This right applies only if: (1) the treatment is on the ground that the worker is a part-time worker; and (2) the treatment is not objectively justified.

The Supreme Court’s (“SC”) decision in Augustine v Data Cars Ltd [2026] UKSC 30 provides welcome clarity on the scope of this protection; must a worker show that their part-time status was the sole reason for the less favourable treatment, or merely an effective cause (i.e. a significant influence) of it?

Background: The legislation 

The PTWR implemented EU Directive 97/81/EC (the “EU Directive”), which concerns the framework agreement for part-term work. The EU Directive sets out that: 

Regulation 5(2) of the PTWR provides that the right for a part-time worker not to be treated less favourably than an employer treats a comparable full-time worker applies:

The differing wording of the PTWR reflects the causation test in discrimination cases under the Equality Act 2010, which covers scenarios where a protected characteristic is an effective cause, but not the sole cause, of the less favourable treatment complained of. Whether Regulation 5(2) should be read in this way (i.e. effective cause), or in a manner consistent with the EU Directive it sought to implement (i.e. sole cause), was the key issue in this case. 

Background: Facts of case

Warren Augustine (“A”) worked as a private hire driver for Data Cars Ltd (“Data Cars”). A worked an average of 34.8 hours per week, fewer weekly hours than typical full-time drivers. Data Cars charged all its drivers, regardless of how many hours they worked or whether they were full or part-time, a fixed weekly circuit fee of £148 to access its booking dispatch system. 

A claimed that the application of the circuit fee was less favourable treatment because of his part-time status, as its effect was that he paid a higher fee per hour than a comparable full-time driver (which resulted him having lower hourly take home pay). 

The ET rejected A’s claim, finding that A was not treated less favourably than a comparable full-time driver - A was charged a circuit fee as it was necessary to enable Data Cars to earn revenue, not because he worked fewer hours than full-time drivers. The ET further found that even if A was treated less favourably on the ground of his part-time status, his claim would fail as the circuit fee was not charged on the ‘sole ground’ that he was a part-time worker. 

A appealed to the Employment Appeal Tribunal (EAT), and later the Court of Appeal (CoA), arguing that: (1) he had been subjected to less favourable treatment; and (2) the wording “on the ground that” contained in Regulation 5(2) should be interpreted to cover the scenario that his part-time status was an effective cause, but not the sole cause, of the less favourable treatment. 

Both the EAT and the CoA found that A had been subjected to less favourable treatment. However, while the EAT and the CoA agreed with A’s interpretation of Regulation 5(2), both courts considered that they were bound by the decision of McMenemy v Capita Business Services Ltd [2007] CSIH 25, which applied a sole cause test. A’s appeals were therefore rejected, with the CoA permitting A to apply to the SC to resolve the sole/effective cause issue.

The Supreme Court’s decision

The SC granted A’s appeal, confirming that for the protection in Regulation 5(2) to apply, the worker must establish that their part-time status was an effective cause of the less favourable treatment. 

The SC held that when implementing the EU Directive through the PTWR, it was open to the UK to relax the narrow “solely because” test and give more favourable protection. The SC found it telling that wording mirroring discrimination legislation rather than the “solely because” wording had been used in the PTWR, and that this was a deliberate choice, with the “effective cause” test the correct interpretation.

So what for employers?

While the SC’s decision focuses on a technical interpretive point, the decision will be of interest to employers who engage part-time workers. In particular:

• Confirmation of test in claims of less favourable treatment: The SC confirmed that the test for causation under the PTWR is the same as in discrimination cases – the worker’s part-time status must be an effective cause of the less favourable treatment complained of, but need not be the sole cause.

This is not only an easier test for workers to satisfy, but also prevents employers from advancing arguments that a claim for less favourable treatment is doomed to fail as there were other reasons together with worker’s part-time status for the less favourable treatment, rather than the worker’s part-time status being the ‘sole’ reason for it.

• Other reasons for treatment relevant to objective justification: However, this does not mean that other reasons for the treatment will not be relevant or considered by the Tribunal. The existence of other reasons for the treatment will likely be relevant to the issue of whether the employer’s treatment can be objectively justified, or the amount of compensation to be awarded. Employers should therefore clearly document and retain evidence of decision-making processes. 

• Potential application for fixed-term workers: Employers who engage fixed-term workers may also wish to take note of the judgment and implications above. The Fixed-term Employees (Prevention of Less Favourable Treatment) Regulations 2002 express a fixed-term worker’s protection from less favourable treatment in an identical way to the PTWR, with the equivalent EU directive also containing identical “sole cause” wording. While not yet confirmed, it can be argued that a similar “effective cause” test should be applied to claims of less favourable treatment brought by fixed-term workers.   

If you would like to discuss any aspect of this decision, please do not hesitate to get in touch with our specialist employment law team.