Researchers found 80 per cent of draught pints have some gas or froth in the head, and 25 per cent fail industry guidelines by being made up of less than 95 per cent liquid.
Draught lager and real ale drinkers will lose a total of pounds 31m because of badly drawn pints over the 12 days of Christmas and the millennium, Camra calculates.
It based the figures on average prices and consumption totals from last year. The pressure group has submitted its evidence to the Government in the hope it will encourage legislation that guarantees the consumer a 100 per cent liquid pint, free from gas or froth.
Iain Loe, spokesman for Camra, said: "There is no legal definition of a pint, which makes it difficult for trading standards to prosecute.
"The reason pints are not filled to the top can be partly blamed on lack of training or carelessness, but there are also some landlords who purposefully try to squeeze more out of each barrel. Industry guidelines specify that if a customer wants a top-up it should be given with good grace.
"But many drinkers don't even ask, or they might have problems catching the barman's attention."
Ian Woolverton, another Camra spokesman, added: "Given that there are about 15 million beer drinkers in the UK, we could enjoy an extra two pints each in our favourite pub if drinkers were given a fair deal.
"It is time the beer lover got a fair deal. The Government must act to protect consumers."Reuse content