Series of underpayments: claiming up to two years of unpaid wages
Acas guidance on claiming roughly up to two years of unpaid wages when underpayments are linked or less than three months apart, contrasted with HMRC's six-year NMW look-back and dual tribunal filing limits.
One short payday hurts. Eighteen short paydays in a row can look like a system. How far back can an employment tribunal go when the underpayment keeps repeating?
Acas's unpaid wages pages set out a worker-facing rule of thumb: you can often claim up to about two years back if either the gaps between underpayments are under three months, or the underpayments are linked (for example the same recurring error). That is Acas guidance, not a personal guarantee that every historic month will be awarded in every pattern. This article keeps that framing careful, separates the filing clock from the look-back, and contrasts tribunal recovery with HMRC National Minimum Wage enforcement (up to six years of arrears on a notice).
In short
- Acas: for unpaid wages you can claim up to 2 years back if gaps between underpayments are under 3 months, or the underpayments are linked.
- Treat that as guidance. Tribunals apply Employment Rights Act 1996 section 23 to the facts; linkage and "series" arguments are fact-sensitive.
- A separate primary time limit still governs when you must start: most claims 3 months minus 1 day from the relevant payday (or last in a series); from 1 October 2026 most become 6 months minus 1 day if the clock starts on or after that date.
- Section 23(4A) also contains a statutory two-year restriction for many unlawful deduction complaints once you present. Filing on time and how far back sums can go are different questions.
- HMRC NMW enforcement can require arrears going back a maximum of 6 years. For the same NMW underpayment you generally choose HMRC or tribunal, not both.
- Some litigation has questioned aspects of the wages backstop; do not treat headlines as settled abolition. Get advice on large historic arrears.
Two clocks, not one
Wage claims trip people up because two ideas share the same "two years" language.
Clock A: when must you start? For most pay and wages claims, Acas says you have 3 months minus 1 day from the date you were not paid correctly. If you were underpaid more than once, that filing clock usually runs from the last time it happened. Acas's time limits page (updated 4 September 2026) adds the dual regime: from 1 October 2026, most claims become 6 months minus 1 day, but only if your time limit starts on or after 1 October 2026. Earlier start dates keep the shorter most-claims rule. Grievance does not pause the clock; notifying Acas for Early Conciliation in time can.
Clock B: how far back can recovery reach? Separately, Acas tells workers that for unpaid wages you can claim up to 2 years back if:
- there is less than 3 months between each underpayment or non-payment, or
- the underpayments or non-payments are linked (Acas example: caused by the same error)
Employment Rights Act 1996 section 23(3) keys series complaints to the last deduction for presentation timing. Section 23(4A) stops a tribunal considering so much of a complaint as relates to a deduction where the wages payment date was before the period of two years ending with the date of presentation (with limited carve-outs in section 23(4B)).
Keep Clock A and Clock B apart. Being "within two years of the old shortfall" does not excuse a late ET1. Being in time on Clock A does not automatically pull every historic month into an award without series / linkage analysis.
Gaps, linkage, and broken chains
GBP example (close gaps): You are 28 and paid monthly. From late 2024 through mid-2026 every payday is short by 8 unpaid closing hours. From 1 April 2026, ages 21+ must get at least £12.71 an hour for NMW / NLW purposes; earlier months use the rate then in force. Eight hours × £12.71 = £101.68 short for a post-April 2026 month. Gaps of about one month sit well under three months. On Acas's guidance wording, that is the kind of sequence that can support a claim looking toward the two-year window, provided you also meet Clock A from the last underpayment.
GBP example (same error): Every week your rota shows 35 paid hours but you work 37 because unpaid cash-up is forced. At £12.71, two unpaid hours = £25.42 a week. Twenty-six consecutive weeks ≈ £661. The error repeats week after week. That matches Acas's "linked / same error" idea. A one-off bonus miscalculation six months earlier may be a separate story.
GBP example (broken chain): Underpaid January and February, fully paid March to August, underpaid again from September. Do not assume January automatically travels on the September filing clock. Protect dates early and get advice if the money is large.
Section 23 in plain English
Section 23 ERA 1996 lets a worker complain about unlawful deductions from wages. For series claims:
- Presentation time (section 23(2)): ordinarily before the end of three months beginning with the payday of the deduction (Acas: 3 months minus 1 day; watch the October 2026 dual regime).
- Series (section 23(3)): timing references point to the last deduction in the series.
- Early Conciliation (section 23(3A) / section 207B): notifying Acas in time can pause / extend the presentation window.
- "Not reasonably practicable" (section 23(4)): late claims are sometimes heard, but Acas says limits are usually strictly enforced.
- Two-year backstop (section 23(4A)): for many deduction complaints, older wages payments fall outside what the tribunal may consider (subject to carve-outs).
That is why Acas can speak about "up to two years" while still insisting on a tight filing deadline from the last wrong payday.
Litigation about the backstop: caution, not slogans
Aspects of the wages two-year backstop have been challenged in litigation. Some commentary has questioned how far the restriction goes and how it interacts with series arguments. Do not treat any named case as settled abolition of the backstop based on headlines, and do not invent holdings. If your arrears stretch toward or beyond two years, or the series pattern is messy: some litigation has questioned the backstop; get advice. Plan on Acas guidance plus the statute as currently in force, then let a qualified adviser map your facts.
Contrast: HMRC NMW look-back (up to 6 years)
If the shortfall is a National Minimum Wage / National Living Wage problem, Acas explains another door: complain to HMRC. HMRC can issue a notice to pay money owed going back a maximum of 6 years, issue fines, and take further action. For the same minimum wage underpayment, Acas says choose either HMRC or an employment tribunal. You cannot take the same issue through two legal processes.
GBP sketch: A 21+ worker underpaid £40 a week below the NMW floor for four years. Rough pool: 52 × 4 × £40 = £8,320. A tribunal path framed by Acas's ~2-year guidance might, on a clean linked pattern, be discussing something nearer £4,160 of that pool (plus hitting Clock A). An HMRC notice route can, in principle, look back up to six years. Exact figures depend on pay reference periods, countable hours, and historic rates. Use the GOV.UK calculator and keep a month-by-month table.
Dual primary time limits still bite
Nothing in the "two years back" guidance replaces Clock A.
| Situation | Filing idea (GB, Acas) | Look-back idea |
|---|---|---|
| Single wrong payday | 3 months minus 1 day (or 6 months minus 1 day if clock starts on/after 1 Oct 2026 for most claims) | That payday plus statute / guidance |
| Several underpayments in a row | Same dual regime from the most recent | Up to ~2 years if gaps under 3 months or linked (Acas guidance; s.23(4A)) |
| Same issue via HMRC NMW | Investigation-led; still act promptly | Up to 6 years arrears on a notice |
Worked dual-regime reminder: Last underpayment payday 20 September 2026 (starts before 1 October): most-claims length stays 3 months minus 1 day → ordinary end 19 December 2026. Last underpayment payday 20 October 2026 (starts on/after 1 October): most claims 6 months minus 1 day → ordinary end 19 April 2027. How far back earlier months go is still the series / linkage / section 23(4A) analysis.
Notify Acas for Early Conciliation inside Clock A if you are heading to tribunal. Notify late and you do not get the pause.
Building a series file
- List every payday, hours paid, hours worked, rate, and shortfall in GBP.
- Mark gaps between underpayments.
- Write one sentence on the common cause.
- Save payslips, rotas, clock exports, and messages.
- Raise the issue in writing (grievance does not stop Clock A).
- Decide HMRC vs tribunal if it is an NMW floor case.
- If tribunal-bound, notify Acas in time, then present ET1.
Common traps
- "Two years means I can wait two years to claim." No. Clock A is months from the last wrong payday (dual regime applies).
- "Acas promised every month for 24 months." It is guidance conditioned on gaps / linkage.
- "A six-month quiet period still counts as one series." It might not.
- "I'll run HMRC and tribunal together for the same NMW shortfall." Acas says choose one process for that issue.
- "Litigation abolished the two-year backstop." Some litigation has questioned it; get advice rather than inventing a holding.
- "Grievance buys me time." It does not change the tribunal limit.
Soft next step
A series claim is only as clear as the spreadsheet behind it. Paye Mes Heures helps you date each underpaid period and total the pounds so you can see whether gaps sit under three months, whether one error repeats, and which payday starts Clock A before you notify Acas. Use that dossier with the Acas unpaid wages pages, then take Early Conciliation or an HMRC NMW complaint as the official next step. For large or old arrears, speak to Acas, a union, or an adviser the same week.