The calculation takes into account the carbon footprint of 40 different sandwiches - homemade and ready-made - and looks at the packaging, how the ingredients are produced, and the food waste that is discarded.
University of Manchester researchers worked out the carbon dioxide equivalent (CO2 eq) for the 11.5 billion sandwiches eaten every year in the UK - according to the British Sandwich Association.
That generates on average 9.5 million tonnes of C02 eq, "equivalent to the annual use of 8.6 million cars", said Professor Adisa Azapagic, from the university's School of Chemical Engineering and Analytical Sciences.
The supermarket-style "all-day breakfast" sandwich - with egg, bacon and sausage - had the worst carbon footprint, estimated to generate 1,441 grams of CO2 eq.
That is the same as driving a car for 12 miles.
Sandwiches containing bacon, ham, sausage, cheese, tomato or prawns were also carbon intensive, according to researchers.
Homemade ham and cheese was the most environmentally friendly sandwich.
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."
Agricultural production and processing of ingredients was the biggest influence on a ready-made sandwich's carbon footprint, accounting for about a third to two-thirds.
Keeping the sandwiches chilled in the shop was the next biggest factor - said to account for a quarter.
Packaging made up 8.5% of the carbon footprint, with transportation 4%.
But making a sandwich at home with the same ingredients could significantly cut its environmental impact by 50%, researchers added.
They said changes to recipes, packaging and waste disposal could help half the carbon footprint of Britain's sandwich consumption.