No one can deny the genius of the marketing behind Pret a Manger. Tapping into our fears about health and junk food, the sandwich chain claims to offer handmade, natural products prepared in a real kitchen not a factory; food that shuns obscure chemicals, additives and preservatives; baguettes “baked throughout the day, the fresher the better”, from “wonderful baker’s ovens” you can see in all its shops. Pret’s claims conjure up a vision of someone else making food for you, almost in the way you would in your own kitchen at home, at a reasonable cost too, and conveniently available on the street corner. It’s everything you could hope for when you grab a sandwich or a salad on the go.

It’s a con, of course, like so much marketing – selling a fantasy that doesn’t quite stand up to scrutiny. The food industry generally prefers to keep the fine details of its inner workings quiet, so it has taken the tragic deaths of two Pret a Manger customers from severe allergic reactions to reveal a few hidden truths about how its products are really made.

Campaigners have been saying as much for years. Real Bread Campaign activists made a complaint to the Advertising Standards Authority in 2016 arguing that Pret’s claims were not accurate, and that it should own up to all the ingredients in its food.

You have to admire the chutzpah of the Pret team that compiled its response. Natural food, shunning additives, was only a mission statement, the team explained, an ideal state not yet fully achieved. And when the company said in marketing that it was “doing the right thing … naturally” it didn’t mean all its food was natural, it merely meant naturally as in “of course”. The ASA quite rightly ruled against the chain on its natural claims, since some of its food contains artificial additives produced from chemical processes – the emulsifiers, extracted gluten, pure starches and preservatives that are essentials in the mass manufacturers’ toolbox for restoring texture to deconstructed commodity parts and ensuring long shelf life.

The comforting bakery ovens might be taken by some to imply that skilled bakers were also present, making bread on their premises from scratch. The ASA doubted people were so naive, and found in the company’s favour on that point. Freshly baked, however, has turned out in Pret’s case to include frozen, part-made bread made in a French factory by a multinational food group with a turnover of close to £900m. This bread has a shelf life of up to a year, and is only finished off in the shops. Supermarkets have been doing this for years too, enticing customers in with the smell of fresh baking in-store of premade, frozen doughs. All this is perfectly legal, since there are no official definitions of these terms. While the Food Standards Agency thinks it is potentially misleading, the ASA thinks it is fine. Either way, it is surely time the legislation was changed to protect consumers.

Meanwhile, caveat emptor – buyer beware. Pret’s meals may be assembled in each shop, but they rely on long, complex international supply chains, with base products that depend on factory processes. While we may find this distasteful, it is the reality of modern industrial food production.

Inevitably it also makes large food retailers’ products vulnerable to contamination or fraud. One of the women died from an allergic reaction to dairy proteins in what was supposed to be a vegan, dairy-free yoghurt dressing. It is not clear how or at what point in the chain the dairy element got there. Its supplier denies responsibility.

Pret is not the first company to fall victim to rogue, undeclared proteins. Protein additives are highly valued by industrial food processors for their many properties – they can emulsify, gel, absorb liquid, and give structure to factory dough. They are often added to chicken destined for fast food, for example, because they prevent it drying out during cooking and retain the cheapest ingredient of all, water. But without full labelling, you don’t know if they are in your food or not. Other companies have been found to be using beef and pork protein additives in poultry in the past.

Concentrated dairy proteins, meanwhile, are commonly used across the industry as stabilisers and emulsifiers. But food allergens are almost always proteins, so their presence without clear labelling is a particularly serious problem.

Pret has been able to take advantage of a waiver of the legal requirement to label with a full list of ingredients when food is sold at the point of production. The secretary of state for food, Michael Gove, wrote to the Green MP Caroline Lucas recently to say that this exemption was made for the “sole purpose of reducing unnecessary regulatory burden on small businesses”. Pret is not a small business. It is an operation worth £1.5bn, owned until recently by the private equity group Bridgepoint Capital, now sold on to JAB, a Luxembourg-based investment fund that also owns Krispy Kreme.

A venture of this size is more than capable of fully listing its ingredients, just as supermarket sandwich and salad sellers do, despite Pret’s argument that the space on its labels is so small, and the number of included ingredients is so large that comprehensive ingredient lists would be impossible. The second woman to die suffered a severe allergic reaction to sesame in the chain’s bread, an ingredient that had not been labelled on the product in store. One man’s regulatory burden is another woman’s exposure to risk of death – it should not have required people to die for the company to come clean, but it has at least said that it will now work towards full labelling.

Perhaps detailed labels will be a spur to achieving that idealised state of its mission statement a bit faster. Will long lists of ingredients blow a hole in the brand? We’ll have to wait and see, but I doubt it. Compared to many of the other fast foods on offer in high streets, Pret’s products have made good acquaintance with fresh ingredients. It has been quick to pick up on people’s desire to cut down on meat and dairy to save the planet by introducing more plant-based meals.

The chain has enjoyed spectacular growth for a reason. To switch to genuinely handmade artisan bread would, as Pret’s chief executive pointed out, cost two or three times as much. With austerity-hit wages, most customers won’t or can’t pay more.

So if the Pret marketing operation is a con, it is a con we want to fall for because our budgets are limited, our time is short, and because shopping defensively, with a list of environmental and health worries, is wearisome. We would rather not think about what it actually takes to supply a chain of over 500 shops at those sort of prices.

Whether this sort of mass-market, globalised supply chain in which meat products remain bestsellers, nothing is out of season and everything is wrapped in throwaway packaging can ever be truly sustainable is another matter. As the UN climate change report said this week, we need to make a huge cultural shift in westernised diets for that.

• Felicity Lawrence is a special correspondent for the Guardian