Note: This advice is given by the CAP Executive about non-broadcast advertising. It does not constitute legal advice. It does not bind CAP, CAP advisory panels or the Advertising Standards Authority.


These are objective claims and are usually synonymous with brand-leading. A number of other claims such as “No.1”, “Premier” and “Leading” will also be interpreted as being akin to a best-selling claim, unless it is clear from the context of the ad that another meaning was intended.

What is required to make a best-selling claim?

The CAP Code requires an advertiser to hold documentary evidence to prove claims that consumers are likely to regard as objective and that are capable of objective substantiation. The nature of the evidence required to substantiate these claims will vary depending on the context in which the claim is made and how it is likely to be understood by its audience, but in all such cases it is likely that comparative evidence relating to unit-sales, market share, or both, will be required.

Whilst data relating to unit sales is likely to be particularly important when making best-selling claims to consumers, in some circumstances it may be more appropriate to look at overall sales turnover figures. In 2014, the ASA held that a claim by a human resources company to be the “UK’s market leader” would need be substantiated via comparative turnover data (Penna plc, 12 February 2014).See Comparisons: General

Can a “best-selling” claim be subjective?

Whilst there may be situations in which the ASA would interpret a “No.1” claim as being the subjective opinion of the advertiser, in practice, such claims will almost always be regarded as objective.

In 2013 the ASA held that evidence relating to awards won by a letting agent, along with information relating to their branches, staff, vehicles, internet data and lettings boards did not substantiate their claim to be “Derby’s No.1 Award Winning Agent” which was considered an objective claim (IMS Lettings Ltd, 17 April 2013). See Property: Sales and board counting and Property: Sales and the use of website data.

Is sales data always required?

Whilst in the majority of cases a “best-selling” claim will need to be supported by sales data, in some circumstances, advertisers may be able to substantiate a “best-selling” claim without holding evidence relating to their competitors’ sales.

The ASA accepted evidence relating to the data collated by an independent third party over a six-month period which compared web traffic with their competitors. The results showed that the number of unique visitors was more than their four closest competitors combined and more than double the number of total visits than their closest competitor. The ASA considered that, while visitor numbers alone was generally not a guaranteed indication of market share, because the difference in web traffic was so great, in these particular circumstances, it was reasonable to infer that they were the leading providing and had substantiated the claim (Medichecks.com Ltd, 14 February 2018).


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