GMB made the call as investigations by newspapers The Sun and The Sunday Mirror over the weekend reported that a high number of pubs across the UK were under-filling pints.
Last week, the union accused publicans of failing to provide consumers with a full pint (100% liquid). The union claims this has become so widespread that commercial contracts between pub companies and tenants are based on yield targets that are impossible to achieve without this practice.
It also claimed that the agreed industry code of 95% minimum, is being encouraged as a maximum liquid in many chains.
The Sun visited 90 pubs across the country and reported that 36 pubs — 40 per cent of those tested served short pints.
However, the reporters conducted their own measurements by pouring the liquid into a measuring jug and noting how much was left once the head had settled.
And according to the Sun report the average shortfall at pubs was 27ml out of a 568ml pint - nearly 5% - within the industry guidelines. Pubs in London and Southampton were all found to be serving full pints.
The Sunday Mirror investigators visited 25 pubs with trading standards officers and reported more than two-thirds were serving underfilled pints. They purchased two pints each at pub chains in Liverpool, Newcastle, Birmingham, London and Cardiff.
All the bars in Birmingham were found to be serving 100% measures while a £3.10 pint of Amstel lager at JD Wetherspoon’s Moon on the Hill in Harrow was 3% overfilled. Yates’s in Harrow served one pint of Foster’s at 5.8% short. In Liverpool one pint of Green King IPA, at JD Wetherspoon’s The Fall Well, was under-filled by 5.5%.
However, the Sunday Mirror report admitted: “We found no evidence of bar staff deliberately pouring too little.”
A spokesman for the British Beer & Pub Association said: “Publicans do not systemically underfill pint glasses. A head on beer, in a traditional British pint glass, is the essence of the British brewing and pub trade.
"We have very clear guidance, that a head on beer shouldn’t be more than five per cent of the volume – but if you want your pint glass filled to the top, publicans are happy to do it, and customers understand this very well.
"It is very hard to see how more regulation on glass sizes would be anything another than a burden to publicans, who have a vast amount of red tape to tackle already.”