The environmental impact of the 11.5 billion sandwiches consumed annually in the UK is the same as around eight million cars, according to scientists.
Researchers at the University of Manchester looked at the carbon footprint of 40 different homemade and bought sandwiches. They considered how ingredients are produced, packaging, and foot waste in order to calculate their environmental impact.
The team then calculated the carbon dioxide equivalent (CO2 eq) – a standard unit for measuring carbon footprints.
The worst culprit was the all-day breakfast sandwich (with bacon, egg, and sausage), estimated to generate 1,441 grams of CO2 eq - the equivalent of driving a car for 12 miles.
Other carbon-intensive fillings included bacon, ham, sausage, cheese, tomato, and prawns.
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 homemade sandwich (ham & cheese) varies from 399-843g CO2 eq per sandwich, depending on the recipe."
The biggest contributor to a sandwich's carbon footprint was found to be agricultural production and processing of ingredients - accounting for 37 percent -67 percent of CO2 eq for ready-made sandwiches.
Packaging and transporting added 8.5 percent and four percent of CO2 eq respectively.
Because of this, making the meal at home cut the environmental impact by up to a half.
Cutting the impact
When it comes to shop-bought sandwiches, the team concluded that changes to recipes, packaging and waste disposal could halve the sandwiches' carbon footprint.
Extending the shelf life could reduce sandwich food waste by around 2,000 tonnes annually.
Prof Azapagic, From Manchester University's School of Chemical Engineering and Analytical Sciences 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."