Britain's annual sandwich consumption produces the same amount of carbon dioxide as more than eight million cars, according to a study published on Thursday.

Around 11.5 billion sandwiches are consumed each year in Britain, half of which are homemade and half bought, according to the British Sandwich Association (BSA).

This annual consumption "generates, on average, 9.5 million tonnes of CO2 eq., equivalent to the annual use of 8.6 million cars," said Adisa Azapagic, professor at Manchester University, which produced the study.

The researchers studied the carbon footprint of 40 different fillings.

Mass-produced sandwiches containing pork (bacon, ham or sausage) were found to have the largest footprint, followed by shop-bought varieties containing cheese or prawn.

The most carbon-intensive filling was found to be the shop-bought "all-day breakfast", comprising egg, bacon and sausage.

Producing this generates 1,441 grams of carbon dioxide, equivalent to the CO2 emissions produced by a 12-mile (19-km) car journey.

The "cleanest" was found to be homemade ham and cheese, with the production, storage, packaging and transport of shop-sold sandwiches accounting for their increased environmental impact.

Researchers estimate that the carbon footprint of sandwiches could be reduced by half by changing recipes and packaging, recycling waste and extending their shelf life.

"We need to change the labelling of food to increase the use-by date as these are usually quite conservative," said Azapagic.

"Given that sandwiches are a staple of the British diet as well as their significant market share in the food sector, it is important to understand the contribution from this sector to the emissions of greenhouse gases," she added.