Under the scheme, to be announced by Prince Charles this week, every stage will be analysed to quantify how much climate-changing gas is released in producing each of the 200 items.

Some restaurants and food producers already display “food miles” — the distance an item has travelled before being sold. But Charles wants Duchy Originals, which is already mostly organic, to go further.

A Clarence House official said the idea was to give consumers the most comprehensive green information available on any product in Britain. Prince Charles wants “people to know the cost of their food in greenhouse gas terms as well as in terms of pounds and pence”, she said.

The prince’s latest green foray follows his instruction to aides last month to use bicycles to help cut down the carbon emissions of his household. Other eco-friendly moves have included experimenting with biodynamics, a form of organic farming that involves planting according to the phases of the moon.

The prince will outline the Duchy Originals plan at a meeting on Wednesday at St James’s Palace attended by business, political, religious and charity leaders, including Rowan Williams, the Archbishop of Canterbury, and Lord Browne, chief executive of BP. Duchy Originals, founded in 1990, makes a profit of about £1m a year, which goes to the prince’s charities.

The company is preparing a “life cycle analysis” of each of its products to quantify the emissions of carbon dioxide, methane, nitrous oxide and other gases blamed for warming the climate.

The quantity could then be printed on the packaging of the products, which range from biscuits and pies to shampoo tested by the prince himself and watering cans “manufactured by skilled craftsmen using traditional techniques . . . hand-dipped in molten zinc alloy”.

It has not yet been decided exactly how the information will be displayed, but it may appear either as a weight of gas or as a percentage of a notional daily “personal carbon allowance” — a concept under development by the government intended to help people adapt their lifestyles to become more eco-friendly.

Attempts have been made to quantify gases emitted in processes such as car manufacturing and even by the average person in their daily life, but analysing food production is a relatively new idea, says Gundula Azeez, policy manager of the Soil Association.

Each breed of animal must be tested for methane emissions and levels of nitrous oxide released through manure. The amount of urine produced by animals and the effects of digging land must be calculated for each farm.

The carbon dioxide from farm lighting and heating will also need to be calculated. Beyond the farm gate, transport, processing and packaging are the main sources of carbon dioxide.

Similarly, for products such as the watering can, the raw materials, production and transport will all be analysed for their gas emissions.

At least a fifth of all greenhouse gas emissions in the UK are caused by food, according to the Food Climate Research Network. Now Charles hopes Duchy customers will be able to use the detailed information on his products’ labels to make “green” choices in the supermarket, which in turn will encourage other food companies to do likewise.

The greenhouse gas emissions of Duchy Original products are likely to vary enormously. Only a third of ingredients are grown at the prince’s Home Farm near Highgrove, Gloucestershire. Ginger for its popular sweet and savoury biscuits comes from Australia, for instance, and its marmalades use oranges from Spain.

Perhaps the most carbon-intensive product is smoked salmon. The fish are caught in Alaska and shipped to Strathaird Salmon, a processor in the Spey valley, where they are smoked over hazel chips from the Duchy of Cornwall’s Herefordshire woodland.

Belinda Gooding, chief executive of Duchy Originals, said: “It is very complex, but it is absolutely the right thing to start trying to do.”

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