Every claim in an advertisement has to be capable of standing up, and the rules that decide whether it does are set out in the CAP Code, the document the Advertising Standards Authority (ASA) uses to judge complaints. This section covers the parts of the Code that come up most often in commercial copy: what counts as misleading, what you need before you make a claim, how comparisons with competitors are treated, and the extra restrictions that apply in finance, health, alcohol and anything aimed at children.
This is a practical summary for copywriters, not legal advice. For definitive guidance consult the CAP Code and the ASA's own resources.
What counts as misleading
An advertisement is misleading if it is likely to deceive the average consumer, or if it leaves out information that consumer would need to make an informed decision. That second half catches more copy than the first. A claim can be true on its own terms and still mislead by omission, if the missing detail would change how the reader reads it.
A subscription described as "£9.99" without stating that the price rises to £24.99 after the first month is a straightforward example. Nothing on the page is false. The reader is still misled, because the figure that matters to their decision has been left off the page. The test is not whether a copywriter intended to deceive. It is whether an ordinary reader, taking the ad at face value, would come away with a wrong impression.
Substantiating your claims before you publish
Any factual claim needs evidence held before the advertisement runs. "Clinically proven", "our best-selling product", "the UK's fastest": each of these is a specific, checkable claim, and each needs a specific, checkable source behind it. If the evidence would not convince a sceptical reader shown it directly, it will not convince the ASA either.
The practical discipline is to write the claim and then ask what document would prove it. If no such document exists, the claim needs softening or removing before publication. This is where vague marketing language often does more harm than plain language: "clinically proven" invites a challenge that "formulated with" does not, because the second makes no claim that needs proving.
Comparative advertising: naming competitors
You can name a competitor and compare your product to theirs, but the comparison has to be about products that meet the same need, on points that can be checked, and it must not create confusion between the two brands. "Cheaper than Competitor X" is fine if it is true and checkable. "Better than Competitor X" needs a defined measure behind the word "better", because otherwise it is an opinion dressed as a fact, and opinions dressed as facts are exactly what the misleading rules exist to catch.
The comparison also has to be fair in what it leaves out. Comparing your product's price against a competitor's while ignoring that theirs includes something yours charges extra for is the same failure as the subscription example above: true in isolation, misleading in context.
Sector-specific rules: finance, health, alcohol and children
Some sectors carry rules beyond the general misleading test, because the potential for harm is higher or the audience is more vulnerable.
- Finance: representative APRs, risk warnings, and the way headline rates are presented are all controlled specifically, because a financial product's true cost is often hidden in the small print.
- Health: claims about treating, curing or preventing an illness need a stronger evidence base than ordinary product claims, and medicinal claims for products that are not licensed medicines are restricted outright.
- Alcohol: advertising must not link drinking to social or sexual success, must not suggest alcohol solves a personal problem, and must not have particular appeal to people under 18.
- Children: advertising that targets children carries tighter rules on what can be claimed, how pressure to buy or to pester a parent is handled, and what imagery and language are appropriate for the age group being reached.
Each of these areas has its own detailed section in the CAP Code, and the detail matters more than the summary. A copywriter working in any of them needs to read the relevant section directly.
Where this guidance sits
None of this replaces reading the Code itself, and nothing here should be read as an assessment of whether a specific piece of copy complies with it. The purpose is to give a copywriter enough grounding to spot the claims that need checking before they go to print or go live, and to know which sector rules to look up when the brief calls for them. For the craft side of writing advertisements once the compliance questions are settled, read the ad copy guide, and you can check a specific headline for clarity and claim strength in the headline scorer. Terms used across both, including CAP Code and USP, are defined in the glossary.