A campaign has been launched to encourage City drinkers to feel confident asking for top-ups on short measure pints.
Trading Standards officers from City of London are recommending that a pint should mean a pint of liquid and drinkers are well within their rights to request a top-up if they want one.
Beer mats have been produced that help consumers check if a pint is short, and drinkers are being encouraged to contact City of London Trading Standards if they are sold a short pint but do not feel confident to ask for a top-up - or if they get an unhelpful reaction when they do.
Trading Standards Officers will also carry out test purchases across the City in the coming months and investigate premises that continue to sell short measures.
The Weights and Measures Act 1985 controls the prescribed quantities that draught beers should be sold in, but there has always been a grey area over whether the head should be included as part of the pint.
Industry body the British Beer And Pub Association says a pint should contain a minimum of 95% liquid and 5% head.
Steve Playle, Trading Standards manager at the City of London Corporation, said: "Consumers are well within their rights to make sure they get exactly what they've paid for.
"It's worth remembering that for a pint costing £5, a shortage of 5% is a 25p cost to the consumer. Drinkers are entirely within their rights to ask for a full pint of liquid if they wish.
"We are reminding people that it is perfectly okay to ask for a top-up whilst stressing that they should continue to drink responsibly."
City of London Corporation Trading Standards has already written to all pubs in the Square Mile to remind them of their legal obligations and to encourage that people with drinking problems are referred to alcohol support services.