The Dispatches programme - aired on Monday, 28 July - analysed the salt, fat and saturates content of sandwiches from five of the UK's largest sandwich companies - Subway, Pret A Manger, Marks & Spencer, Boots and Greggs - and ranked them according to the FSA's traffic light system.
Greggs ranked bottom in terms of fat, with 69% of its sandwiches getting a red light for fat content. Subway was the worst for salt content, with 93% of sandwiches tested earning a red light.
The investigation revealed that Marks & Spencer's British Oakham Chicken and Pancetta Caesar sandwich contained nearly 45g of fat, which is more than is found in a McDonald's Big Mac and chips.
Reported by Channel 4 News presenter Alex Thomson, the undercover investigation included secretly filmed footage of unhygienic practices at one factory that supplies small shops and garages. The footage showed an employee replacing a sandwich that had fallen on the floor and a pest-control expert telling workers that their hygiene standards were "disgusting".
When asked if he would eat a sandwich made at the factory, an independent expert on food safety and environmental health said: "Absolutely not. I couldn't trust them not to give me food poisoning."
The programme also examined the amount of information available about the ingredients on the packaging of sandwiches and the marketing techniques used in the £5bn sandwich industry.