Sandwiches are not always a healthy fast-food option: some contain more fat than two McDonald's cheeseburgers; others contain as much salt as several packets of crisps.

Sandwiches from five of the UK's largest sandwich companies – Subway, Pret a Manger, Marks and Spencer, Boots and Greggs – were examined by Channel 4's Dispatches programme.

The investigation, which will be broadcast next week, found that Marks and Spencer's British Oakham Chicken and Pancetta Caesar sandwich contained almost 45 grams of fat – nearly a third more than is found in a Big Mac and chips.

M&S said it had open and clear labelling on its food and it offered diet versions of its chicken sandwiches.

The programme's researchers measured the levels of fat, saturates and salt in 100 sandwiches against the traffic light system created by the Food Standards Agency (FSA), designed to help consumers. A red light warns that a product contains high levels of "something we should be trying to cut down on", according to the FSA.

Products with more than 1.5g of salt per 100g are given the red rating. A product is given a red light for fat if its fat content is more than 20 per cent. For saturated fats, that level is 5g in every 100g.

Greggs came bottom in terms of fat, with 69 per cent of its sandwiches earning a red light. Pret a Manger was only slightly better, with 68 per cent.

Boots finished top, with just 14 per cent of its sandwiches tested earning a red light for their fat content. M&S ranked second, with 47 per cent of the sandwiches tested earning a red light.

Subway, the UK's fastest growing sandwich chain, came bottom of the list in terms of salt content, with 93 per cent of its sandwiches tested getting a red light. The company's 12-inch meatball Marinara was found to contain 9.4g of salt – the equivalent of 18 packets of ready-salted crisps.

Subway said it was "working closely with suppliers and the Food Standards Agency to achieve 2010 salt targets". The FSA aims to limit salt in sandwiches to 1.3g by the end of the decade.

An undercover investigation by Dispatches also found poor levels of hygiene at the factory of a small, independent sandwich maker, KMB caterers. An employee was filmed replacing a sandwich that had fallen on the floor, while a pest-control expert is seen telling workers: "Basically your hygiene standards are disgusting. If an environmental officer came here today they'd close you down right now."

KMB said yesterday it had not been allowed to see the film. Its solicitors said the company has been in operation since 1986 and takes standards of hygiene very seriously.

Dispatches: Sandwiches Unwrapped will be shown on Channel 4 at 8pm on Monday July