The campaign is part of a move to provide a more ethical way of shopping and backs up findings from online research company YouGov, which revealed how ethics and health issues are increasingly determining what products customers decide to buy.
The campaign, comprising ads in the national press, contain headlines such as: "It's not just our green dyes that won't harm the environment", "We're committed to reducing salt faster than you can say 'sodium chloride'", "Our ready meals lack a certain something. Hydrogenated fats" and "After these sandwiches have disappeared, so will the packaging".
It will also run in more than 420 Marks & Spencer stores and on the side of Marks & Spencer lorries.
As well as promoting its new range of Fairtrade clothing, which became available in the UK at the end of 2005, the campaign is featuring messages and imagery about M&S products and its health, quality and environmental aspects.
The move follows the retailer's decision to cut the salt content of its sandwiches by 15% over the last year and by being ranked top retailer for responsible fishing by Greenpeace and the Marine Conservation Society.
The YouGov survey, which interviewed 2,300 people, found that a third of respondents said they had decided not to buy an item of clothing because they felt concerned about where it had come from or under what conditions it had been made.
Some 59% said they had avoided buying a food product due to similar concerns and 78% said they would like to know more about the way clothes are made, including the conditions in the factories where they come from and the use of chemicals in their manufacture.
Stuart Rose, chief executive of Marks & Spencer, said: "Customers want good value, but they care more than ever how food and clothing products are made. 'Look behind the label' is the first time we've talked about the lengths we go to ensure everything we sell is produced in a responsible way. Our customers increasingly want to know about this, which is why we've decided to tell them what we stand for."
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