The CT600 Filing Gap at Companies House: How 1.2 Million Corporation Tax Filers Resolve From Britain's 5.5 Million on the Register — and What the Public Accounts Reveal About the Rest

The arithmetic nobody runs

Britain's company register at Companies House holds roughly 5.5 million entities. HMRC's corporation tax statistics, published each autumn, report approximately 1.2 million CT600 returns processed in the preceding financial year. The arithmetic is immediately striking: roughly 4.3 million companies on the public register file no corporation tax return at all.

The conclusion this invites — that four-fifths of UK companies are somehow outside the tax net — is wrong. The gap is real, but it is structural, not evidential. Every one of those 4.3 million non-filers falls into a defined category that the Companies Act 2006, the Corporation Tax Act 2010, and HMRC's own administrative practice have carved out. Knowing those categories matters, because the absence of a CT600 says something specific about a company — and the public record at Companies House supplies the clues.

The five places the non-filers sit

A UK-resident company is chargeable to corporation tax under section 5 CTA 2010 on all its profits wherever arising. The obligation to deliver a return, however, is triggered by HMRC issuing a notice to deliver a company tax return under paragraph 3 of Schedule 18 to the Finance Act 1998. If HMRC never issues the notice — typically because it has accepted the company is dormant for CT purposes — no CT600 is due. That administrative filter, combined with entity classification, explains most of the gap.

Here is where the 4.3 million fall:

1. Dormant companies (roughly 1.4 million). A company that has never traded since incorporation, and files dormant accounts (AA02) or abbreviated dormant accounts at Companies House, may notify HMRC that it is dormant for CT purposes. HMRC generally accepts this notification and withholds the notice to deliver. Dormancy is the single largest category and accounts for approximately one-quarter of the total register — a figure that has held broadly steady since at least 2018. The registrar can now challenge a dormant filing under ECCTA if the company shows indicia of trading elsewhere.

2. Entities not within the charge to CT. Limited liability partnerships do not file CT600s; they are transparent for tax purposes and their members pay income tax or capital gains tax on their share of profits. Companies limited by guarantee that are registered charities may qualify for exemption from CT on their primary-purpose trading income under Chapter 3 of Part 11 CTA 2010. Community interest companies, industrial and provident societies, and certain other non-standard entities also fall wholly or partly outside the CT600 population.

3. New incorporations within their first accounting period. Companies House incorporated roughly 800,000 new companies in the 2024–25 year. A company incorporated on 1 April 2026 will not reach its first corporation tax filing deadline until 1 January 2028 — twelve months after the end of its first accounting period. At any point in time, a meaningful slice of the register simply has not yet been required to file. The first tell is on the public register itself: no accounts yet due, no filing history beyond the IN01.

4. Dissolution-pending companies. A company in the process of voluntary strike-off (form DS01) or compulsory strike-off (Gazette notice under section 1000 of the Companies Act 2006) will have filed its final accounts and its last CT return. It remains on the public register — and therefore in the 5.5 million — for approximately three months between Gazette notice and dissolution, during which it files nothing. Companies House flags these entities with a "Dissolved" or "Active — Proposal to Strike off" status visible on the beta service.

5. Non-trading companies that have not filed for dormancy. Some companies incorporate, register for CT, file at least one return, then cease trading without formally entering dormancy or dissolution. If HMRC has already issued a notice to deliver, the company must continue filing nil returns until HMRC accepts cessation. A small residual population sits in this gap — Companies House accounts may show a static or declining balance sheet with no turnover, but no formal dormant filing has been made.

What the public accounts actually reveal about tax

The iXBRL-tagged full accounts filed at Companies House include a note on taxation — typically Note 4 or Note 5 in the standard FRS 102 layout — that discloses the current tax charge for the period, a reconciliation between the tax charge and the profit before tax multiplied by the applicable CT rate, deferred tax movements, and prior-period adjustments.

For a user of the public register, that tax note is the single most useful data point for estimating a company's taxable profit. If a company files full (non-abbreviated) accounts, the note lets you work backwards from the stated current tax charge to an approximate taxable profit figure. The exercise is imprecise — group relief, brought-forward losses, R&D relief, and the patent box all wedge between accounting profit and taxable profit — but it is often the only quantitative tax signal available on the public record.

The gap between what the public accounts show and what a CT600 contains is substantial:

In short: the public accounts give you the headline tax charge and a broad reconciliation. The CT600 gives HMRC the granular computation. For third parties conducting counterparty due diligence, the accounts offer a useful but incomplete picture — and the thinner the accounts (FRS 105 micro-entity, abbreviated, or dormant), the less tax information the public register holds.

Where the audit threshold cuts across

The April 2025 uplift in the small-company and audit-exemption thresholds (the regulations amending section 382 of the Companies Act 2006) increased the audit-exemption turnover threshold to £15 million and the balance-sheet threshold to £7.5 million. That moved a large cohort of medium-sized companies — those with turnover between £10.2 million and £15 million — out of the mandatory audit requirement.

The relevance to corporation tax is straightforward: unaudited accounts carry less assurance that the tax note — including the current tax charge and the reconciliation — is stated accurately. A preparer who understates the tax charge in unaudited accounts filed at Companies House is not necessarily understating the CT600 filed with HMRC, but the public-record signal is weaker. For the analyst using Companies House data to estimate a company's tax position, the audit threshold is the line between "reasonable starting point" and "use with caution."

Why the gap matters for due diligence

For anyone using the public register to assess counterparty risk — a credit analyst, a procurement officer conducting supplier due diligence, or a compliance team working through a corporate network — the CT filing status of a company is an informative signal that costs nothing to check:

• Dormant company filing AA02 every year. Low risk on its face, but verify that the dormancy is genuine. A company that files dormant accounts while actually trading commits an offence under section 1112 of the Companies Act 2006. The economic purpose of such an entity may be to obscure beneficial ownership rather than to trade. Cross-check against the PSC register and any online trading presence.

• Company filing full accounts with a meaningful current tax charge. The tax note provides a cross-check on reported profitability. If the current tax charge implies a profit materially different from the P&L figure — allowing for the standard CT rate and common adjustments — ask why. Large permanent differences between accounting profit and taxable profit should be visible in the reconciliation note.

• Company filing micro-entity accounts (FRS 105). The tax note in FRS 105 accounts is materially thinner than in FRS 102 — often a single-line "tax on profit" entry with no reconciliation. For the very companies that make up the majority of the register, the public record offers the least tax transparency.

• Company with no accounts yet due (first filing period). No negative signal, but also no public financial data. Due diligence on new incorporations must rely on director history, PSC data, and the stated SIC code — not on accounts.

• LLP. The LLP's own accounts show profit allocations to members but not the tax paid by them. To understand the tax position of an LLP, you need each member's self-assessment — none of which is on the public record. An LLP with substantial declared profits and no publicly visible tax payments is not avoiding anything; it is simply taxed at the member layer.

ECCTA and the data horizon

The Economic Crime and Corporate Transparency Act 2023 does not directly amend corporation tax filing obligations — those sit in tax legislation administered by HMRC, not in the Companies Act 2006. But ECCTA changes the data environment around the non-filing population in two ways.

First, the new registrar powers to query and strike off companies with inaccurate or suspicious filing profiles (sections 80 to 85 of ECCTA) mean that a company claiming dormancy at Companies House while showing indicia of trading elsewhere — a website, a business bank account, active invoices — faces a higher probability of registrar challenge than it did two years ago. The dormant-company population may therefore contract at the margins as the registrar exercises its enforcement tools.

Second, the identity verification regime for directors and PSCs, once fully rolled out, raises the barrier to incorporating a company solely to hold an asset while keeping the beneficial owner off the register. A company that never files a CT600 because it never trades — but that holds a UK property via a nominee arrangement — will face greater scrutiny of its director and PSC filings under ECCTA's verified-identity architecture. Companies House and HMRC data remain separate statutory domains, but the identity verification bridge makes it harder to run a company through the register without leaving a traceable human footprint.

Neither development closes the CT600 gap, which is structural and administrative rather than evidential. But both change what the gap means in practice: the non-filing population is becoming a harder place to hide, and the public record is becoming a more reliable tool for distinguishing the genuinely dormant from the deliberately opaque.