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.

“Best-selling” claims, and other claims which are likely to have the same meaning to consumers, such as “leading” and “no.1” claims, are likely to be considered objective comparisons with identifiable competitors.

The CAP Code requires advertisers to hold documentary evidence to substantiate claims that consumers are likely to regard as objective and that are capable of objective substantiation. Additionally, any comparisons with identifiable competitors must be verifiable and comply with other relevant comparisons rules (3.33 – 3.40). See also Comparisons: Identifiable competitorsComparisons: General and Types of Claims: “No. 1”.



Is sales data always required?


The CAP Code requires advertisers to hold documentary evidence to substantiate claims that consumers are likely to regard as objective and that are capable of objective substantiation.

The nature of the evidence required to substantiate “best-selling” claims, or any claim which is likely to be understood in the same way by consumers, will vary depending on the context in which the claim is made and how it is likely to be understood by its audience. However, it is likely that comparative evidence relating to unit-sales, market share, or both, will be required.

Absolute best-selling claims are likely to be understood as an entire market comparison, and advertisers would therefore need to demonstrate that they are the best-selling across the entire market, taking into account all competitors.

The ASA considered that the claim “Richmond, proud to be the nation’s favourite” was likely to be understood as a best-selling claim by consumers. Because the advertiser could demonstrate that, at the time the ad appeared, Richmond Sausages was the best ranked and best-selling sausage in Great Britain, the claim was not likely to mislead (Kerry foods Limited, 25 October 2017).

The ASA upheld a complaint about the claim “#1 holiday website on the planet”, because the advertiser did not provide evidence to demonstrate that the it had the largest turnover of holiday rentals when compared to their global competitors (Rental Republic Ltd, 19 January 2022). In contrast, the claim “Europe’s number one airline” was not considered misleading, because the advertiser could demonstrate that,  over a reasonable period before the ads were produced, it had carried more passengers than any other European airline (Ryanair Ltd, 7 February 2018).

Marketers must ensure that the way data is collected for their own sales and competitor sales is sufficient to allow for an accurate comparison. The claim “ST. MORIZ UK’S NO. 1 TAN” was considered misleading because it was not clear if the advertisers’ sales figures were accurate, or if the data had been collected, presented, and applied in a different way to their competitors. In addition, the data had been subject to masking, whereby some retailers did not provide their volume of sales figures for certain products, including St Moriz’s own product, as it was commercially sensitive. Therefore, the data was considered unreliable (Hothouse Partnerships Ltd, 18 April 2018.


The CAP Code requires that comparisons with identifiable competitor products “must objectively compare one or more material, relevant, verifiable and representative feature of those products” (rule 3.35).

Marketers do not need to identify explicitly the competitor or product that they are comparing with to be subject to the rules on comparisons with 'identifiable' competitors, and the ASA’s interpretation of ‘identifiable’ competitors is broad. If a consumer can identify at least one competitor that is being compared, whether or not it is identified explicitly in the ad, this will mean that the comparative claims must be verifiable. In most cases, “no.1” claims are likely to be understood as comparisons with identifiable competitors and must be verifiable.

The information required to make a claim verifiable will depend entirely on the specific comparison and the evidence used to support it. Generally speaking, marketers should include as much information as possible in the ad to ensure that consumers are able to check it for themselves, and include a signpost in the ad to information on the basis of the comparison. If verifying the comparison requires specialist knowledge, consumers should be able to get a knowledgeable and independent person or organisation to verify the comparison for them.

The ASA considered that the claim "#1 World's Live-Games Provider" would be understood as a best-selling claim. Because the ads did not direct businesses to any additional information regarding the comparisons and explaining the basis for the claims (ASTOK Ltd, t/a TVBet, 15 April 2020).

See Comparisons: Verifiability.

Is sales data always required?

Whilst in most cases best-selling claims need to be supported by sales data, in some circumstances, advertisers may be able to substantiate these claims without holding evidence relating to their competitors’ sales, as long as they are able to provide evidence which effectively demonstrates that they are the market leading.

The ASA accepted data collated by an independent third party over a six-month period which compared an advertiser’s web traffic with that of its competitors. The results showed that the number of unique visitors was more than its four closest competitors combined and more than double the number of total visits than its 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 the advertiser was the leading provider and had substantiated the claim ( Ltd, 14 February 2018).

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