Something quietly changed at HMRC over the past few years. The letters landing on doormats across Britain, the ones asking pointed questions about your income, your expenses, your side hustle, didn’t always come from a human inspector who’d noticed something odd. Increasingly, they came from an algorithm. HMRC’s AI tax investigation tools have been expanding at pace, and while the Revenue insists they’re catching genuine fraud, a growing number of honest, self-employed people are finding themselves on the wrong end of an automated red flag they had absolutely no idea was coming.
I’ve spoken to freelancers, sole traders and small business owners who describe the experience as bewildering. One graphic designer in Leeds received an inquiry letter asking her to explain a three-year discrepancy in her declared income that turned out to be entirely down to a legitimate bad year followed by a very good one. Another, a self-employed plumber in Bristol, was flagged for having expenses that looked “anomalous” compared to others in his sector. Both cases were eventually resolved in the taxpayers’ favour. Both still caused months of stress and professional fees for an accountant to sort out.
How HMRC’s AI compliance system actually works
HMRC’s system is called Connect. It’s been running in some form since around 2010, but the machine learning capabilities layered on top have grown considerably. Connect pulls in data from an enormous range of sources: Companies House records, Land Registry, letting platforms, bank transaction data shared under international agreements, social media, online marketplaces like eBay and Vinted, and even information from the DVLA. It then cross-references all of that against what you’ve declared on your tax return.
If the algorithm detects what it considers an inconsistency, that triggers a risk score. High enough score, and your file gets flagged for a compliance check, which can range from a simple nudge letter asking you to review your return, all the way up to a full formal investigation. HMRC processes around 300,000 compliance checks a year, and the proportion initiated by automated tools rather than human judgment has risen sharply. The Revenue doesn’t publish an exact figure for what percentage of flags are AI-generated, but tax professionals I’ve seen quoted in the trade press put the estimate well above half.
Why false positives happen more than you’d think
The problem with any algorithm built to detect outliers is that genuine outliers exist. A musician who earned £8,000 in 2023 and £47,000 in 2025 because they finally got a sync deal is going to look suspicious to a system trained on smooth income curves. A childminder who suddenly has very low declared income one year because she took maternity leave is going to look like she’s hiding earnings. The system is good at spotting patterns. It’s not good at understanding context.
There’s also a data quality problem. Connect is only as reliable as the information fed into it. If a third-party platform reports your earnings incorrectly, or if a previous employer files a P60 with an error, that discrepancy goes into the system as a potential problem with your return. You might not even know the wrong data exists until the investigation letter arrives.
Tax adviser groups including the Low Incomes Tax Reform Group have been raising concerns about this for several years. Their research consistently shows that lower-income self-employed people, who are less likely to have professional accountants and more likely to make innocent record-keeping errors, are disproportionately affected by automated compliance activity. You can read more about their work at litrg.org.uk, and I’d genuinely recommend it if you’re self-employed and feeling confused about your obligations.
What rights do you have if HMRC flags you?
Here’s the bit that most people don’t know, and it matters. Being flagged by HMRC’s system does not mean you’ve done anything wrong, and you have meaningful rights throughout any compliance process.
First: you do not have to respond to an informal nudge letter by simply agreeing with whatever HMRC suggests. These letters often invite you to “consider whether your return is correct” and to make a voluntary amendment if you think there’s been an error. If your return was correct, you can say so, clearly, in writing.
Second: if HMRC opens a formal inquiry under Section 9A of the Taxes Management Act 1970, you have the right to know what they’re looking at, to be represented by an accountant or tax adviser, and to challenge their conclusions at a Tax Tribunal if you believe they’re wrong. The process is adversarial in the legal sense, meaning HMRC has to make their case, not just assert it.
Third: if the investigation finds nothing wrong, you can claim some of your professional costs back in certain circumstances, though this is unfortunately not automatic and requires a formal claim.
The general advice from tax professionals is: don’t panic, don’t ignore it, and get professional help early. The longer an inquiry runs without a clear response, the more complicated it tends to become.
The gig economy is particularly exposed
If you’re earning money through platforms, whether that’s driving for a delivery service, selling crafts on Etsy, renting a room through a lettings app, or doing shifts through a care agency app, you’re in a category HMRC is paying very close attention to. The Digital Platforms reporting rules, which came into force at the start of 2025, now require UK-based platforms to report user earnings directly to HMRC annually. That’s a huge amount of new data flowing into Connect, and a lot of people who’ve been quietly earning below what they assumed were reporting thresholds are about to find out that HMRC knows more than they did.
This is also where the line between innocent confusion and actual avoidance can get genuinely blurry. Some people earn through multiple gig platforms simultaneously and simply don’t realise the combined total tips them over the Self Assessment threshold. The AI doesn’t know whether your ignorance was deliberate. It just knows the numbers don’t match.
If you’re ever trying to make sense of whether a contact from HMRC is legitimate or part of a wider pattern of suspicious official-looking communications, it’s worth checking resources like 0lly.uk which covers this sort of thing in detail. Fake HMRC phishing letters are unfortunately common, and distinguishing them from genuine compliance correspondence is a skill worth having.
The rise of AI in tax compliance is, on balance, probably a good thing for catching serious fraud. But the cost of that efficiency, measured in stress, accountancy bills and wasted hours for people who did nothing wrong, deserves more public scrutiny than it currently gets. If you’re self-employed, keep your records clean, understand your obligations, and don’t assume that a clean conscience means you’re immune to an automated flag. The algorithm doesn’t care about your intentions. It just cares about the numbers.
For anyone who’s curious about how much of their financial life is now being quietly tracked and cross-referenced, the piece we did on subscription costs Brits don’t realise they’re paying touches on just how many platforms are now sharing data with regulators. And if you’ve ever wondered how far automated systems can reach into everyday working life, the article on AI screening tools used by UK employers is a useful companion read. Algorithms are everywhere now. HMRC’s version just has particularly sharp teeth.
Frequently Asked Questions
How does HMRC decide who to investigate for tax compliance?
HMRC uses a system called Connect, which pulls data from Companies House, Land Registry, online platforms, banks and other sources and compares it against your declared income. If the algorithm detects what it considers an inconsistency, it generates a risk score. High-risk cases are flagged for a compliance check, which may be handled automatically or passed to a human inspector.
What happens if HMRC sends me a compliance check letter?
Don’t ignore it and don’t automatically agree with whatever the letter implies. If it’s an informal nudge, you can respond in writing confirming your return is correct. If it’s a formal inquiry under Section 9A of the Taxes Management Act, you have the right to professional representation and to challenge HMRC’s conclusions at a Tax Tribunal if needed.
Can HMRC see my earnings from gig economy platforms like Deliveroo or Etsy?
Yes. Digital Platforms reporting rules introduced in early 2025 require UK-based platforms to report user earnings to HMRC directly each year. This means income from delivery work, craft sales, room rentals and similar sources is now automatically shared with HMRC and fed into their compliance system.
What is a false positive in an HMRC AI investigation?
A false positive is when HMRC’s algorithm flags someone for investigation who has actually done nothing wrong. This happens when legitimate income patterns, like a bad year followed by a very good one, or a career break, look statistically unusual to the system. HMRC doesn’t publish figures on how often this occurs, but tax professionals estimate it’s common among lower-income self-employed workers.
Can I claim my accountancy costs back if HMRC investigates me and finds nothing?
In some circumstances, yes. If HMRC opens a formal inquiry and closes it without finding any underpayment, you may be able to claim professional costs through a formal application. This isn’t automatic, so you’d need to raise it explicitly. Tax investigation insurance, which many accountants offer as an add-on, is worth considering before any inquiry begins.