Retrospective Legislation, Proration Of Tax Rates, And The Fragmentation Of Accounting Periods: Revisiting The Federal High Court Decision In Shell v. FIRS

1. Introduction

The decision of the Federal High Court ("the Court") in Shell Petroleum Development Company of Nigeria Limited v. Federal Inland Revenue Service (FIRS)¹ has brought renewed attention to fundamental principles of Nigerian tax law, particularly regarding the proration of tax rates within an accounting year and the retrospective application of tax legislation.

The case arose from a mid-year increase in the Tertiary Education Tax (TET) rate introduced by the Finance Act 2023 and raised important questions regarding the proper treatment of tax liabilities during periods of legislative transition. At its core, the dispute required the Court to determine whether different tax rates could apply to different segments of the same accounting year, or whether the applicable rate must be determined solely by reference to the law in force at the end of the accounting period.

Beyond the immediate dispute between the parties, the decision has broader implications for taxpayer certainty, statutory interpretation, and the administration of tax reforms in Nigeria, particularly in light of the ongoing implementation of the new tax framework.

This article analyzes the facts, arguments, the court's reasoning, comparative study from other jurisdictions on similar subject matter and the broader implications of the decision.

2. Facts of the Case

Shell Petroleum Development Company of Nigeria Limited self-assessed its TET liability for the 2023 accounting year by applying:

• a 2.5% TET rate to profits earned from January to August 2023; and 
• a 3% TET rate to profits earned from September to December 2023, in line with the commencement date of the Finance Act 2023.

The FIRS, however, assessed Shell's TET liability using the 3% rate for the entire 2023 accounting year, arguing that the applicable tax rate is the rate in force at the end of the accounting period and that the accounting year cannot be fragmented for tax purposes.

3. Arguments Presented

Shell argued that:

• the increased TET rate should apply only from 1 September 2023, when the Finance Act 2023 became effective;
• tax statutes should not be applied retrospectively unless the legislature clearly expresses such intention; and
• Shell had a vested right to the lower TET rate of 2.5% for the period prior to the commencement of the new law.

FIRS' Position

The FIRS maintained that:

• the accounting period is a fixed statutory framework that cannot be subdivided for tax purposes;
• the applicable tax rate is the rate in force at the end of the accounting period;
• annual taxes such as TET are not subject to proration within the year; and
• there is no legal basis under the TET Act, Companies Income Tax Act (CITA), or Petroleum Profits Tax Act (PPTA) for fragmenting an accounting period for the purpose of applying different tax rates.

The Court was therefore called upon to resolve these competing interpretations and determine the correct legal position.

4. Decision of the Court

The Federal High Court decided in favour of the FIRS and held that:

• assessable profit must be computed for the entire accounting period as a single unit;
• there is no statutory or judicial basis for dividing an accounting year into segments for tax computation;
• TET is chargeable strictly on the basis of a full accounting period and is not subject to proration;
• the taxpayer had no vested or accrued right as of 1 September 2023; and
• the application of the 3% rate to the entire year constituted prospective, rather than retrospective, application of the law.

In essence, the Court concluded that, since TET is assessed annually, the applicable rate of TET is the rate prevailing at the end of the accounting period.

While the Court's decision resolved the immediate dispute between the parties, it also raised broader legal and doctrinal questions regarding the nature of vested rights, the interpretation of accounting periods, and the limits of retrospective tax legislation.

5. Legal and Doctrinal Issues Arising from the Decision

a) Fragmentation of Accounting Periods

The decision in Shell v. FIRS appears to suggest that an accounting period is invariably a fixed twelve-month period from 1 January to 31 December and that fragmentation of an accounting period for tax purposes is generally impermissible.

Respectfully, this interpretation may not fully reflect the structure of existing tax legislation.

There are several statutory circumstances in which accounting periods may be fragmented, including commencement of business, cessation of business, change in accounting date etc.

Similarly, the above submission by the Court appears to contradict the wordings of section 29(6) of the CITA which permits apportionment of profits between periods in circumstances where it is necessary to undertake such apportionment.

b) The Doctrine of Vested Rights

The doctrine of vested rights protects accrued rights from retroactive legislative interference and promotes legal certainty, one of the foundational canons of taxation.

The Supreme Court in Afolabi v. Governor of Oyo State² established that a statute does not retrospectively abrogate vested rights without clear legislative intention; and legislation is presumed not to impair proprietary rights unless expressly stated.

Similarly, in Adesanoye v. Adewole³, the Supreme Court held that a statute is retrospective where it takes away or impairs a vested right, creates a new obligation, imposes a new duty and attaches a new disability to past transactions.

Courts have historically been reluctant to interpret statutes in a manner that removes accrued rights without clear legislative direction.

The Court in Shell v. FIRS, however, concluded that the taxpayer's duty had not crystallized because the accounting year had not ended and therefore characterized the duty as a contingent liability/expectation rather than a vested duty.

This reasoning represents a significant doctrinal shift and may have implications for taxpayer certainty during periods of legislative transition.

c) Retrospective Application of Tax Legislation

The decision in Shell v. FIRS has been cited in some quarters as authority for retrospective application of tax laws, particularly in relation to recent tax reforms. This interpretation, however, requires careful clarification.

The Court in its judgement did not endorse retrospective taxation. Rather, it held that tax liability for an annual tax crystallizes at the end of the accounting period and that the applicable law is the law in force at that point in time.

Accordingly, the decision does not support the application of legislation enacted after the end of an accounting period to earlier periods.

For example, the Nigerian Tax Act has a commencement date of 1 January 2026. Based on the principle established in Shell v. FIRS, the Act cannot be applied retrospectively to the 2025 accounting period. It can only apply to accounting periods that are completed subsequent to its commencement, i.e. accounting periods ending after 1 January 2026.

It is important to note that these doctrinal questions are not unique to Nigeria. Courts and tax systems in other jurisdictions have grappled with similar issues, particularly in situations involving

mid-year legislative changes and transitional tax provisions. We have assessed some of the court decisions made in other jurisdictions below. We have also briefly discussed the decision of a Federal High Court in Nigeria on a similar subject matter.

6. Comparative Analysis of Other Jurisdictions as well as court decisions in Nigeria

1. United Kingdom

In the United Kingdom, the principle that tax legislation should not apply retrospectively without clear statutory authority is well established.

In Huitson v. HMRC (2011)⁴, the UK Court of Appeal upheld retrospective tax legislation but emphasized that such measures are permissible only where parliament clearly expresses retrospective intent; and the measure serves a legitimate public interest.

Similarly, UK tax law routinely provides statutory mechanisms for the apportionment of income where tax rates change during a financial year. Transitional provisions are typically included to ensure fairness and predictability in the tax system.

These mechanisms demonstrate a deliberate legislative approach to managing mid-year changes in tax rates rather than leaving the issue to judicial interpretation.

2. India

Indian tax jurisprudence also reflects a cautious approach to retrospective taxation. In CIT v. Vatika Township Pvt Ltd (2014),⁵ the Supreme Court of India held that tax legislation is presumed to be prospective unless the legislature clearly expresses a contrary intention.

The Court further emphasized that retrospective taxation should not impose unexpected burdens on taxpayers in respect of completed transactions.

Indian tax statutes frequently include detailed transitional provisions designed to address changes in tax rates, commencement of legislation, allocation of income across periods. These provisions are intended to minimize uncertainty and reduce litigation.

3. Nigeria

The Accugas case⁶ has been cited as authority for protection of vested rights, and authority against retrospective legislation. In the Accugas case, the Plaintiff sought to protect his accrued rights under CITA prior to subsequent amendment by the Finance Act 2019. The Court in its ruling held that "where any right, privilege, or obligation is accrued to an individual/entity under an enactment, same must be saved despite the repeal of such enactment." This decision aligns with the locus classicus cases of Afolabi V Gov of Oyo state and Adesanoye V Adewole.

7. Lessons for Nigeria

The experience of other jurisdictions highlights three important policy considerations for Nigeria. a) Clarity in Transitional Provisions

Where tax rates change during an accounting period, legislation should explicitly address whether proration is required, how income should be allocated, and the effective date of the new rate. This reduces disputes and enhances taxpayer certainty. Given that the Nigerian Tax Act became effective in January 2026, we expect that the Nigerian government will expeditiously issue a circular providing clarity on the above.

b) Protection of Taxpayer Certainty

Predictability is essential to voluntary tax compliance and investment planning. Uncertainty regarding the application of tax laws during transitional periods may increase disputes, discourage investment and undermine confidence in the tax system. We therefore recommend that the government in issuing clarifying circulars aligns its position with international best practices.

Footnotes

1. Suit No FHC/L/CS/2341/24

2. (1985) 2 NWLR 734

3. (2000) JELR 43864 (SC)

4. [2011] EWCA Civ 893

5. CIVIL APPEAL NO.8750 OF 2014 (arising out of SLP (C) No. 540 of 2009)

6. Accugas V FIRS & anor suit No: FHC/ABJ/CS/1289/2020

The opinion expressed in this article is solely personal and does not represent the views of any organization or association to which the authors belong.