Costa, the coffee shop chain owned by Whitbread, could find itself in hot water over its claim in its advertising campaign that “7 out of 10 coffee lovers prefer Costa”.

The Advertising Standards Authority (ASA) is understood to have concluded in a draft report that Costa’s campaign fails to pass muster on a number of points, although its final recommendations are not known.

The complaint to the ASA, which was made by Starbucks, centres on the claims made by Costa in the ad campaign, launched in March, based on a survey of 334 coffee shop users. In one version, used on posters, national newspapers and online, it declares: “Sorry Starbucks — the people have voted.”

Starbucks, which has stepped up efforts to rid itself of negative consumer perceptions, declined to comment ahead of the ASA adjudication. However, the watchdog confirmed that it had received a complaint from the American coffee behemoth arguing that the Costa adverts were misleading and misrepresented the basis of the claims.

Experts said that the campaign gave the impression that both its shops and its coffee were preferred by drinkers and it was only in the smallprint that Costa pointed out that the “7 out of 10” claim related only to a Costa cappuccino beverage compared to a Starbucks or Nero cappuccino.

The only part of the complaint that has been made public is Costa’s use of the claims in radio sponsorships, which was referred to Ofcom by the ASA. In it, Starbucks argued that its rival’s campaign “could be understood as a preference claim against all coffee drinks or even more widely as a claim against all aspects of its business”, including store ambience.

Starbucks also pointed out to Ofcom, which upheld its complaint, that cappuccino was “a drink that represents just a small proportion of coffee sales”, adding that for a survey to be properly representative, it would need to compare Costa against the whole market, whether independent or chains.

Unlike its radio sponsorships, Costa’s print campaign does qualify its claim in the small print, although this does not appear to have saved it from at least a degree of censure in the ASA draft.

The ASA Executive’s recommendation is being considered by the ASA Council, which can seek expert advice before reaching its conclusions. The council is likely to decide in the next few days if there has been a breach of the advertising codes.

Costa declined to comment last night, although it is believed to be confident that it will not be asked to withdraw the ads, which were produced by the Karmarama agency and were based on research undertaken by Tangible Branding, the marketing consultancy.

The chain, which has almost 1,000 stores in the UK, recently reported like-for-like sales growth of 2.5 per cent in the six months to August 27, citing the campaign as one factor in its success.