Inconsistent and confusing labels on best-selling sandwiches on the high street are making it difficult for shoppers to make meaningful comparisons and choose the healthiest options, a consumer group has warned.

Retailers such as Tesco, Aldi, Caffe Nero and Greggs are criticised by Which? for failing to print so-called 'traffic light labelling' on the front of packs which claims that portion sizes and nutritional content vary so much that consumers could be eating three times as much fat and double the amount of salt as the same sandwich bought elsewhere.

Which? researchers looked at the calorie, fat, saturated fat and salt content of three of the most popular pre-prepared sandwiches – chicken salad, egg mayonnaise and bacon, lettuce and tomato (BLT) on sale at supermarkets and coffee chains.

But they found that fat and salt content varied widely while inconsistent labelling across stores meant that healthier sandwich options were not always obvious. The traffic light labelling system uses a colour-coded wheel of red, amber and green symbols to indicate levels of salt, fat and other nutrients but is not compulsory in the UK. Six out of the 15 retailers it compared include the traffic light system, but the rest do not.

Of the inconsistencies singled out by Which? are a Morrisons chicken salad sandwich contains 11.7g fat (amber/medium) compared with one from Waitrose which contains 6.0g fat (green/low). Waitrose uses traffic lights, whereas Morrisons doesn't. A Lidl BLT has 3.36g salt (red/high) but one from Boots has 1.5g salt (amber/medium). Again, Boots uses traffic lights while Lidl doesn't. And an Aldi egg mayonnaise sandwich contains 22.3g fat (red/high) and one from Asda contains 10.1g (amber/medium). Asda uses traffic lights, Aldi doesn't.

This week, the government's Food Standards Agency (FSA) launched a consultation on nutrition labelling in Northern Ireland and Scotland.

Which? executive director Richard Lloyd said: "With obesity levels reaching epidemic proportions, it's more important than ever that consumers know exactly what they're eating. Many retailers are already using traffic-light labelling, but the rest need to catch up and do what works best for consumers. We want to see the government insist that all food companies use traffic lights on their labels, so there's a clear, consistent system that makes it easier for people to make informed choices about what they eat."

A spokesperson for the FSA said: "New EU regulations on food labelling were introduced at the end of last year that require manufacturers and retailers to make many changes to their food labels. While providing front-of-pack information is voluntary under the regulations, every company that does so has to provide information about calories alone, or calories plus the amount of fats, saturated fat, sugars and salt."

A spokesperson for Aldi said: "There is currently a debate about which system of food labelling is best for consumers. We offer our customers the opportunity to make an informed choice by providing Guideline Daily Amount (GDA) labelling as we currently feel that this is the best way of keeping our customers informed. We will continue to review the situation as we approach the implementation of the food information regulation in 2014. As a responsible business, we will continue our work to reduce salt and saturated fats in our food."