Analysts from Citigroup said that the campaign, which includes fairtrade, sustainability and animal welfare, had given M&S a lead of at least six months over Tesco, Asda, Sainsbury's and Morrisons in persuading shoppers that it is the most ethical place to shop.
In a note, Citigroup said: “We expect the company to use this advantage to introduce new areas of 'responsibility', particularly in areas where its competitors will not be able to follow.”
It added: “While it is impossible to split out the impact of any one individual campaign on sales performance, the evidence collected by the company indicates that this was the most positive campaign the business has ever run, and measured, on brand perception.”
Earlier this month, as the multiples announced plans to remove hydrogenated fats from their ready meals, M&S ran full-page adverts in national newspapers to highlight the fact that its ready meals were already free of them.
It also launched a range of fairtrade clothing earlier this year.