The overall annual consumption of sandwiches across the UK has the same environmental impact as the use of around eight million cars a year, scientists have claimed.
Researchers arrived at the figure after studying the carbon footprint of 40 different types of sandwiches - both home-made and pre-packaged - taking account how the ingredients were produced, the packaging, as well as food waste discarded at home and elsewhere in the supply chain.
The team from the University of Manchester calculated the carbon dioxide equivalent (CO2 eq) - a standard unit for measuring carbon footprints - for the snacks, based on British Sandwich Association (BSA) figures that 11.5 billion sandwiches are eaten each year in the UK.
Professor Adisa Azapagic, from the university's School of Chemical Engineering and Analytical Sciences, said: "Consuming 11.5 billion sandwiches annually in the UK generates, on average, 9.5 million tonnes of CO2 eq, equivalent to the annual use of 8.6 million cars."
The ready-made "all-day breakfast" sandwich containing egg, bacon and sausage was found to have the highest carbon footprint, generating 1,441 grams of CO2 eq - which is the same as driving a car for 12 miles.
Those containing pork meat (bacon, ham or sausages), cheese, tomato or prawns were also deemed as carbon-intensive by the researchers but home-made favourite ham and cheese was found to have the lowest CO2 eq.
Writing in the journal Sustainable Production and Consumption, the team said: "The estimated impact from ready-made sandwiches ranges from 739g CO2 eq for egg & cress to 1,441g CO2 eq for the bacon, sausage & egg option.
"The carbon footprint of the most popular home-made sandwich (ham & cheese) varies from 399-843g CO2 eq per sandwich, depending on the recipe."
Agricultural production and processing of ingredients were found to be the largest contributor to a sandwich's carbon footprint, accounting for around 37%-67% of CO2 eq for ready-made sandwiches, with packaging material coming in 8.5% of CO2 eq, and transporting and refrigerating adding a further 4%.
Keeping sandwiches chilled in supermarkets and shops accounts for up to a quarter of their greenhouse gas emission equivalent, the researchers said, adding that making them at home using the same ingredients could reduce carbon emissions by 50%.
The team concluded that a combination of changes to the recipes, packaging and waste disposal could halve the carbon footprint of the sandwiches.
According to the BSA, extending the shelf life of sandwiches can help reduce 2,000 tonnes of sandwich waste annually.
Prof Azapagic said: "We need to change the labelling of food to increase the use-by date as these are usually quite conservative.
"Commercial sandwiches undergo rigorous shelf-life testing and are normally safe for consumption beyond the use-by date stated on the label.
"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."