UK Finance (No. 2) Act 2023, Parts 3–4 — Multinational Top-up Tax and Domestic Top-up Tax (Pillar Two / GloBE)

Mechanism

Legal basis and OECD lineage

Finance (No. 2) Act 2023 (c. 30) is the UK's primary legislative implementation of the OECD/G20 Inclusive Framework Pillar Two GloBE Model Rules (published December 2021 / Commentary March 2022). Unlike the EU's approach — which uses a binding directive (Council Directive 2022/2523) requiring Member State transposition — the UK enacted its own stand-alone statute in domestic-law style, rewriting the GloBE mechanics into UK statutory language rather than cross-referencing OECD model text. Section 121 of the Act declares this intent explicitly:

> "The purpose of this Part is to implement the provisions of the Pillar Two rules relating > to top-up tax."

The Act creates two new UK tax charges:

Part 3 — Multinational Top-up Tax (MTT / IIR-equivalent)

The MTT charges UK members of qualifying MNE groups (consolidated group revenue ≥ EUR 750 million in at least 2 of the 4 preceding fiscal years) when the group's jurisdictional ETR in any jurisdiction falls below 15%. The UK parent or "responsible member" pays the top-up charge for the entire UK sub-group's proportionate share of the under-taxed profit.

Key computational mechanics (faithful to GloBE model rules, re-expressed in UK statute):

• GloBE ETR = GloBE Covered Taxes ÷ GloBE Net Income (jurisdictional aggregation)
• Top-up Amount = (15% − GloBE ETR) × Excess Profit (GloBE Net Income minus

Substance-Based Income Exclusion for payroll and tangible assets)

• UK Inclusion Ratio = UK group member's proportionate share of the top-up (allocated

by ownership percentage, consistent with OECD's Inclusion Ratio mechanics)

Part 4 — Domestic Top-up Tax (DTT / QDMTT)

The DTT applies the same 15% GloBE ETR floor to UK-located constituent entities of in-scope MNE groups, collecting top-up domestically before any foreign IIR (including EU Member State IIRs or the MTT itself) can apply. As a QDMTT:

• It satisfies the IIR top-up obligation for UK-located entities — a foreign UPE's IIR

cannot charge top-up on UK entities if the UK DTT has already collected the full 15% floor.

• Each UK-located constituent entity is generally separately chargeable (no "responsible

member" concept), though a single member may be elected to be liable for the group's DTT.

• HMRC is the collecting authority.

The DTT ensures that top-up revenue on UK operations stays in UK fiscal receipts rather than being ceded to a foreign UPE state's IIR or to EU Member States applying UTPR.

UTPR addition (Finance Acts 2024/2025)

The original Act deliberately omitted the UTPR backstop, mirroring Canada's approach of implementing IIR + QDMTT first. The UTPR was subsequently enacted through Finance Act 2024 and Finance Act 2025 amendments, taking effect for accounting periods beginning on or after 31 December 2024. This completes the full three-rule GloBE stack under UK law.

Transposition approach: "domestic-style redraft"

The UK chose not to transpose the OECD model rules verbatim. Instead, it rewrote the GloBE mechanics into standard UK income-tax statute syntax, integrating with existing UK tax administration (HMRC, CRS/CbCR infrastructure, self-assessment regime). This approach:

• Maintains alignment with the OECD model rules in economic substance
• Gives UK courts domestic statutory language to interpret (rather than OECD Commentary)
• Creates a reference model for non-EU common-law jurisdictions — Korea adopted a

structurally similar direct-codification approach via AITA Chapter V

Relationship to the broader Pillar Two architecture

Downstream implications

• UK competitiveness for MNE HQ location: Pre-Pillar Two, the UK's 25% statutory

corporation-tax rate (raised from 19% in April 2023) was already above the 15% floor for UK-source income. The MTT/DTT primarily affects UK-parented MNEs with subsidiaries in sub-15%-ETR jurisdictions (IE, CH, SG, HK, AE, LU), forcing top-up on profits routed there.

• Irish subsidiaries of UK groups: Ireland (12.5% CT rate) adopted its own QDMTT in late

2023, so IE-located profits of UK-parented MNEs will face Irish QDMTT rather than UK MTT — the top-up is collected by Ireland, not the UK. This limits the MTT's practical revenue impact on UK MNEs with significant Irish operations.

• US-parented MNEs operating in UK: The DTT (QDMTT) means that top-up on UK operations

of US-parented MNEs is collected by HMRC. The US CAMT (15% corporate AMT) is not a qualifying IIR, so US parents cannot offset CAMT against UK DTT; the DTT is a genuine additional tax cost on UK-located profits for US MNE groups where UK ETR falls below 15%.

• Interaction with UK R&D tax credits and Patent Box: HMRC guidance confirms that UK

non-refundable tax credits (including Patent Box) reduce GloBE covered taxes and thus GloBE ETR, potentially triggering top-up unless the ETR stays above 15% on a jurisdictional basis. UK-located R&D-intensive companies with a high Patent Box benefit may be pulled below 15% on UK-source income.

• UTPR and US MNEs post-2024: With the UTPR operative from FY 31 December 2024, UK

entities of US-parented MNE groups face UTPR exposure where the US group has under-taxed income not covered by a qualifying IIR in the UPE jurisdiction. This creates friction in the UK-US investment relationship and is the subject of ongoing OECD-level discussions about UTPR application to groups headquartered in non-implementing jurisdictions.

• Finance (No. 2) Act 2023 as a post-Brexit unilateral standard-setter: The UK enacted

its Pillar Two legislation independently of the EU directive timeline, demonstrating capacity to implement complex multilateral tax frameworks outside EU legislative cycles — a recurring theme alongside UK CBAM (Finance Act 2026) as independent implementation of multilateral frameworks.

Open questions

• US non-reciprocity: The US has not enacted an IIR or a qualifying UTPR. UK application

of UTPR to US-parented MNEs continues despite US Treasury objections; diplomatic resolution through OECD Subject-to-Tax Rule or a bilateral safe harbour arrangement is unresolved.

• GloBE ETR computation divergence: With 40+ jurisdictions enacting Pillar Two through

varied domestic-law channels, computational divergence in GloBE ETR methodology (covered-tax timing adjustments, deferred-tax treatment, CbCR safe harbour application) is a growing audit risk. HMRC and EU Member States may reach different ETR conclusions on the same MNE.

• Transitional safe harbour sunset (post-2026): The Transitional CbCR Safe Harbour

(Simplified ETR / Routine Profits / De Minimis tests) reduces compliance burden through FY 2026. Post-2026 full GloBE computation will raise UK MNE compliance costs materially.

• UTPR coordination with future US legislation: If the US enacts a GloBE-qualifying IIR

(via a GILTI reform satisfying OECD conditions), the UK UTPR would no longer apply to US-parented MNEs — a significant structural change to the architecture as it stands today.