This guide is written for solicitors, trainees and law students in England and Wales.
Part-time workers’ rights changed materially on 12 August 2026. In Augustine v Data Cars Ltd [2026] UKSC 30 the Supreme Court held that part-time status need only be an effective cause of less favourable treatment to breach the Part-Time Workers (Prevention of Less Favourable Treatment) Regulations 2000, and disapproved the sole-ground test from McMenemy v Capita that tribunals had followed since 2007. This guide explains the framework as it now stands and what to audit.
Who is this guide for?
Employment lawyers advising on either side of the new test, HR and reward teams reviewing pay and charging structures, and practice managers in firms and chambers where fixed contributions, rents or per-head charges apply across staff working different hours. Around a quarter of the UK workforce works part time, so few employers sit outside the exposure. The underlying dispute concerned a £148 weekly circuit fee charged to every driver regardless of hours, which is exactly the kind of uniform charge the ruling now reaches.
What do the 2000 Regulations provide?
Regulation 5 of the 2000 Regulations gives a part-time worker the right not to be treated less favourably than a comparable full-time worker as regards contractual terms or by being subjected to any other detriment. The comparator must be a real one under regulation 2(4): employed by the same employer under the same type of contract, engaged in the same or broadly similar work, normally at the same establishment. Treatment is assessed applying the pro rata principle unless it is inappropriate, and an employer escapes liability only where the treatment is justified on objective grounds. A worker who suspects a breach can request a written statement of reasons under regulation 6, and complaints go to the employment tribunal within three months. None of that architecture changed in Augustine. What changed is causation, the gateway to all of it.
What is the effective cause test?
Treatment is “on the ground that” the worker is part-time where part-time status was an effective cause of it, even if other causes operated too and even if it was never the main reason. Lady Simler reasoned that this phrase carried a clear and authoritatively established meaning in domestic discrimination law when the Regulations were made, and that the drafters deliberately declined to copy the narrower “solely because” wording of the European framework agreement, so the established jurisprudence governs. McMenemy, which required sole causation and bound the EAT across Great Britain, is no longer good law anywhere in the UK. The practical consequence is stark for uniform charges. An employer’s evidence that a flat fee applies to everyone, for reasons unconnected to hours, no longer answers the claim, because uniformity against different hours is precisely what generates the pro rata disadvantage. Our news report on Augustine v Data Cars covers the litigation history in full.
What should you audit?
- Flat or fixed charges applied to all staff regardless of hours: circuit fees, equipment or desk charges, chambers rents, per-head contributions to practising costs or insurance.
- Allowances and benefits paid at a uniform rate rather than pro-rated, or gated behind hours or earnings thresholds that part-time staff cannot realistically meet.
- Overtime triggers, bonus eligibility rules and benefit waiting periods expressed in full-time terms.
- The evidence base for objective justification: a legitimate aim, and proportionality of the uniform structure against pro-rated alternatives, documented before a claim rather than assembled after one.
- Existing advice and precedents that state the sole-ground test, which need correcting.
- Comparator exposure: identify which full-time staff a part-time worker would point to under regulation 2(4).
Common mistakes
The recurring error before Augustine was treating uniform application as a complete defence, and it now fails at the first hurdle. A second mistake is assuming justification is easy: cost alone rarely justifies discrimination, so a flat structure needs a better rationale than administrative convenience. Third, advisers sometimes reach for the wrong comparator rules, importing Equality Act hypothetical comparators into a regime that requires an actual one. Fourth, employers overlook that detriment claims cover more than pay, reaching training access, shift allocation and career progression. The government’s part-time worker guidance remains a useful plain-language baseline for the unchanged parts of the regime, and statutory reform is arriving alongside the case law, tracked in our Employment Rights Act implementation guide.
Relevant official sources
The judgment is on Find Case Law as [2026] UKSC 30, with the case details on the Supreme Court’s website. The Part-Time Workers (Prevention of Less Favourable Treatment) Regulations 2000, SI 2000/1551, are on legislation.gov.uk as amended.
Date last updated: 13 August 2026.