Legislation agreed by the European Parliament will allow the UK to continue its use of the mile for roads, the pint for draught beer, cider and bottled milk, and the troy ounce for precious metals. The opt-out has been granted indefinitely.
But drinkers and publicans are being warned by Lancashire County Council's Trading Standards officers to avoid "metrication by stealth".
County Coun. Marcus Johnstone, Cabinet member for community, planning and partnerships, said: "Surveys conducted by Trading Standards in recent years have revealed a large head of an inch can easily amount to 10% of the drink, thereby making the measure closer to a half litre than the traditional pint. So, the message is - if you think there is not enough beer in your glass, ask bar staff to top it up.
"Contrary to some reports, there are no other changes to metric quantities. People may be under the impression that they will see a complete return to old-style imperial system of pounds and ounces, but this is not the case.
"Most goods, from pre-packed goods to petrol, will be sold in metric quantities. However, to preserve our traditions, labelling to show the imperial weight or measure can now continue indefinitely and will not be phased out."
Lancashire's Chief Officer Trading Standards Officer, Jim Potts, warned: "Most beer glasses are a pint to the brim and a small head is normal. However if the head is large, the resulting amount of beer could be nearer half a litre.
"The current code of practice relating to the sale of beer by the pint allows up to 5% 'short measure' on beer served in brim glasses to allow for the head of foam and expects consumers to ask for a top up if desired.
"We do carry out checks on what is served at the pumps as issue advice and warnings wherever necessary."