The advert refers to a recent appeal won by Molson Coors against an HMRC claim that it owed more than £50m in unpaid duty.
The company won the appeal by arguing it did not need to pay due to a reformulation to manufacture Carling at a lower strength, while continuing to label the beer as 4% ABV.
The Tennent's advert alleges: “In September 2012 Molson Coors brewing company reduced the ABV of their Carling lager in order to save millions of pounds in duty.
“Since then they have been misleading customers and consumers by continuing to display the ABV of Carling on the packaging as 4%.”
'Not very honest'
“At a tax tribunal in February/March 2017 Mr Philip Rutherford, vice president of tax for Molson Coors Europe, confirmed that Molson Coors took the deliberate decision not to change the labelled ABV in order to stop customers ‘demanding a slice’ of the savings. That’s not very honest is it?”
It also offers customers the chance to “switch to Scotland’s favourite (honest) lager” by calling Tennent's.
Molson Coors declined to comment on the advert. However, in September, a spokesperson for the company told The Morning Advertiser: “Recently there have been several inaccurate reports which have oversimplified and misinterpreted a very complex legal case which Carling won on all counts in April this year.
“Beer does vary slightly in actual ABV from brew to brew - as with many consumables, there are fractional variances permitted by law. The law allows for different methodologies in the calculation of duty, as was confirmed by the recent tribunal decision.
Modern brewing an 'exact science'
“We account for duty calculated on the actual ABV of Carling when it is in the Bright Beer Tank, prior to it reaching the filling lines where the liquid is then packaged. This method of duty calculation means we bear all duty on beer losses after that point, including for example, losses from drainage, spillage and evaporation in the packaging process."
However, several breweries cast doubt on the claim that small ABV variances were a normal part of large-scale beer production.
A representative from a large-scale international brewery, who did not want to be named, said Molson Coors had been “accurately inaccurate” in the production of Carling to save money on duty.
“If they wanted to produce a 4% beer, then within +/-0.1% they would be able to do that,” they said. “Modern brewing is a relatively exact science. But if you're not targeting 4% you can be 'accurately inaccurate' and as a result save money on duty.”