Dairy farmers and British MEPs are fighting a last-ditch campaign to block the moves to harmonise packaged food across the European Union.

Under the rules, which replace British imperial measures with a European metric system, the pint-sized milk carton would be cut to half a litre. But with 68 millilitres fewer in the pack, consumer groups fear the price will remain the same — short-changing customers.

The same law will affect the traditional British loaf because, unlike many other European countries, the UK packages and slices its bread.

Bakers say it would force them to abandon the standard sizes of “small” and “large” in favour of various EU sizes. This could not only add to costs but also confuse consumers.

The moves are likely to annoy British shoppers, who pay an extra £20 a week on supermarket bills because of the common agricultural policy.

British grocers and shoppers are already subject to EU laws that force them to display fruit and vegetables in metric measures.

The legislation is set to go through by a majority in the parliament in Strasbourg on Thursday, despite amendments tabled by the UK Conservatives.

The plans have come about because the European commission wants to harmonise EU rules on pre-packaged food quantities that prescribe the size of packs in which some types of food — such as milk, butter and bread — must be sold.

Chris Heaton-Harris, a Tory MEP, said: “These are unnecessary measures which will just end up confusing people or making them angry. I am in favour of protecting consumers but in this case the best way to offer that protection is to stick with what consumers know best.

“Traditions are important to us in Britain and I can quite understand people getting upset when they see those traditions under threat.”

Gordon Polson, director of the Federation of Bakers, said the old law was “an essential consumer protection measure and ensures that we keep the traditional British loaf”.

Dairies said the cost of replacing bottled pints would be prohibitive. Kirby & West, a family-run dairy business delivering milk and other products to 60,000 doorsteps in and around Leicester, said: “If we were unable to use the pint bottle at all it would basically be the end of the doorstep delivery.”

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