The UK tax gap for 2024-25 is £59.2 billion, equal to 6.4% of all tax that should have been paid. The tax gap is the difference between the tax HMRC theoretically should collect and what it actually receives. Small businesses are the single largest contributor at 62%, and the biggest cause is honest carelessness, not deliberate evasion.
Those figures come from HMRC's Measuring Tax Gaps 2026 edition, published on 23 June 2026. HMRC collected £865.2 billion in 2024-25, which is 93.6% of all tax due, leaving the 6.4% gap. This article explains why small firms dominate the gap, what HMRC is doing about it, and the practical steps that keep you on the right side of a compliance check.
What is the UK tax gap and how big is it in 2026?
The tax gap is the shortfall between the tax that should be paid in theory and the tax actually paid. For 2024-25, HMRC estimates that shortfall at £59.2 billion, or 6.4% of total theoretical tax liabilities. Put the other way round, the UK pays 93.6% of the tax it owes.
The gap is not one single problem. It is the sum of underpayments across every tax and every type of taxpayer, from a sole trader who fat-fingers a figure on their return to large-scale criminal fraud. Understanding the breakdown is what tells you where the real risk sits, and for small businesses the answer is uncomfortable.
Why are small businesses 62% of the tax gap?
Small businesses are the single largest group in the tax gap, accounting for 62% of it in 2024-25. That share is unchanged on the prior year but has climbed from 58% in 2020-21. Mid-sized businesses, by contrast, make up about 7%.
The reason is not that small firms are uniquely dishonest. It is a question of volume and complexity: there are millions of small businesses, most without an in-house finance team, each filing returns across Income Tax, Corporation Tax, VAT and PAYE. Small errors at that scale add up to a very large number. Around half of the small-business tax gap is Corporation Tax, which reflects how easy it is to get company profit calculations and allowable expenses slightly wrong.
Is the tax gap caused by evasion or honest mistakes?
This is the part that surprises most owners. The largest single cause of the tax gap by behaviour is failure to take reasonable care at 35%, followed by error at 16%. Deliberate evasion is only 12%. In other words, roughly half the gap is honest carelessness and genuine mistakes, not fraud.
| Behaviour | Share of the tax gap (2024-25) | What it means |
|---|---|---|
| Failure to take reasonable care | 35% | Not keeping proper records or not checking figures |
| Error | 16% | Genuine mistakes despite reasonable effort |
| Evasion | 12% | Deliberately hiding income or inflating costs |
| Other causes | Remaining 37% | Includes non-payment, avoidance, criminal attacks and the legal interpretation gap |
The takeaway for a small business is clear. You do not need to be cutting corners to end up in the tax gap. Sloppy bookkeeping, a forgotten invoice, or a guessed figure is enough. The good news is that the same point cuts the other way: because the dominant causes are carelessness and error, the gap is largely fixable with better record-keeping.
Which taxes make up the biggest part of the gap?
By tax type, Corporation Tax is 35% of the total tax gap, the largest single slice. That matters enormously for small limited companies because around half of the entire small-business tax gap is Corporation Tax. Getting your company profit figure right, claiming only genuinely allowable expenses, and applying the correct rate is where most of the risk concentrates.
| Customer group | Approximate share of the tax gap |
|---|---|
| Small businesses | 62% |
| Mid-sized businesses | About 7% |
| Large businesses, individuals, criminals and others | Remaining 31% |
If you run a company, read our complete guide to Corporation Tax in the UK and our practical notes on how to reduce Corporation Tax legally so the savings you claim are ones HMRC will stand behind.
What is HMRC doing about it and how does Connect target small firms?
HMRC is investing heavily in compliance, and small businesses are the explicit focus because that is where the largest share of the gap sits. The main tool is Connect, HMRC's data-matching system. HMRC cannot freely browse your bank account, but Connect cross-references your returns against a vast pool of third-party data: bank interest reports, Land Registry records, card-payment data, and digital-platform reports under the DAC7 rules covering marketplaces and apps.
If your declared income does not match what the data says, Connect flags the discrepancy and can prompt a compliance check. We explain the boundaries of HMRC's access in can HMRC check your bank account without your permission, and what tends to spark an enquiry in what triggers an HMRC investigation. If you do receive a letter, our overview of the HMRC compliance check process walks through what to expect.
How does an honest error become part of the tax gap? (Worked example)
Here is how a genuine mistake by an honest sole trader turns into both a contribution to the tax gap and a real bill.
Imagine Priya, a self-employed graphic designer. When she files her Self Assessment return she accidentally omits a £4,000 invoice she was paid late in the year. She is a basic-rate taxpayer, so that omitted income should have generated £800 of Income Tax (20% of £4,000), plus Class 4 National Insurance, which we will set aside to keep the example simple.
- The gap: that missing £800 of tax is exactly the kind of underpayment HMRC measures. It is not evasion, it is "failure to take reasonable care", the single biggest category in the gap.
- How HMRC spots it: the client who paid the £4,000 claimed it as a business expense, so Connect sees money flowing to Priya that does not appear on her return.
- The penalty: because the error reduced the tax she paid, HMRC can charge a penalty calculated as a percentage of the tax lost. Penalties for careless errors are lower than for deliberate ones, and an unprompted disclosure reduces them further, but a penalty still applies on top of the £800.
- Interest: HMRC charges interest on the £800 from the date it was originally due until she pays, so the longer it goes unnoticed, the more it costs.
One forgotten invoice has now cost Priya the original tax, a penalty and interest. Had she also filed or paid late, the fixed Self Assessment penalties below would have stacked on top.
What does a late or wrong return actually cost?
Self Assessment penalties are mechanical and they bite even when no tax is owed. Miss the 31 January online deadline and you get an immediate £100 fixed penalty. The longer you leave it, the worse it gets.
| How late | Late-filing penalty |
|---|---|
| The moment you miss 31 January | £100 fixed (even if no tax is due) |
| After 3 months | £10 per day, up to 90 days (maximum £900) |
| After 6 months | The greater of £300 or 5% of the tax due |
| After 12 months | A further £300 or 5% of the tax due, whichever is greater |
Paying late is charged separately: a 5% late-payment penalty on tax still unpaid at 30 days, again at 6 months, and again at 12 months, plus interest throughout. For a full breakdown, see our guide to the Self Assessment late-filing penalty and our notes on HMRC interest and penalties for late payment. If you genuinely cannot pay, HMRC's online Time to Pay arrangement is available where you owe under £30,000 and meet the criteria; see HMRC Time to Pay for Self Assessment.
How can small businesses stay out of the tax gap? (Compliance checklist)
Because the gap is driven by carelessness and error, good systems are your best defence. Work through this checklist:
- Keep digital records and prepare for MTD for Income Tax. Making Tax Digital for Income Tax becomes mandatory from 6 April 2026 for sole traders and landlords whose qualifying income (gross turnover before expenses) is over £50,000, assessed on the 2024/25 return. The threshold drops to over £30,000 from 6 April 2027 and over £20,000 from 6 April 2028. It requires digital records and quarterly updates via compatible software. Our MTD for Income Tax guide explains who is in scope.
- File and pay on time. Treat the 31 January deadline as immovable to avoid the £100 fixed penalty and everything that stacks behind it.
- Reconcile regularly. Match your bank statements to your bookkeeping every month so nothing slips through, exactly the kind of omission in Priya's example. See bank reconciliation for small business and the bookkeeping basics.
- Get your Corporation Tax figures right. Since CT is around half of the small-business gap, take care with profit calculations and allowable expenses. The 19% small-profits rate applies to profits up to £50,000, the 25% main rate to profits of £250,000 and over, with marginal relief in between.
- Declare all income, including side income. Connect and DAC7 platform data make undeclared earnings easy to spot. If you have a side hustle, read the HMRC side hustle tax rules.
If you are worried about an existing error, a discrepancy, or a letter from HMRC, our tax investigation services can manage the process and the disclosure for you, which usually reduces both the penalty and the stress.
What should you do next?
The 2026 tax gap figures send small businesses a simple message: accurate records and on-time filing now matter more than ever, because HMRC is investing in compliance and aiming squarely at firms like yours. Most of the risk is honest error, which means most of it is preventable. Get your bookkeeping tidy before MTD lands, reconcile monthly, and never guess a figure on a return.
If you would like a second pair of eyes on your records or your filings, book a free call with Zmartly and we will help you tighten up before HMRC has a reason to look.
Frequently asked questions
What is the UK tax gap for 2024-25?
The UK tax gap for 2024-25 is £59.2 billion, equal to 6.4% of total theoretical tax liabilities. It is the difference between the tax that should be paid in theory and what HMRC actually collected, which was £865.2 billion or 93.6% of all tax due. The figures come from HMRC's Measuring Tax Gaps 2026 edition, published on 23 June 2026.
Why are small businesses the biggest part of the tax gap?
Small businesses make up 62% of the tax gap, the single largest group, because there are millions of them filing complex returns without dedicated finance teams. The dominant causes are failure to take reasonable care (35%) and error (16%), not evasion (12%). Around half of the small-business gap is Corporation Tax.
Can HMRC catch a small error on my tax return?
Yes. HMRC's Connect system cross-references your return against third-party data such as bank interest, card payments and digital-platform reports under DAC7. HMRC cannot freely browse your bank account, but if your declared income does not match the data, Connect can flag it and trigger a compliance check, with penalties and interest on any underpaid tax.
When does Making Tax Digital for Income Tax start?
MTD for Income Tax is mandatory from 6 April 2026 for sole traders and landlords with qualifying income over £50,000, based on the 2024/25 return. The threshold falls to over £30,000 from 6 April 2027 and over £20,000 from 6 April 2028. It requires digital records and quarterly updates through compatible software.
