Marks & Spencer's ethical campaign 'Look behind the label' has been the most successful it has ever run, according to a City brokerage.

The campaign, launched this year, focuses on the way M&S sources and makes products, highlighting everything from use of toxic-free clothes dyes and salt reduction in ready meals to animal welfare.

It was the first major campaign to concentrate on the way products are sourced and made and its success reflects the consumer trend for a more ethical way of living and healthier lifestyle.

The campaign, under the leadership of chief executive Stuart Rose, has embedded the concept of M&S as a responsible retailer, Citigroup said in a broker note.

"The evidence collected by the company indicates that this was the most positive campaign the business has ever run, and measured, on brand perception," Citigroup said.

By challenging consumers to 'Look behind the label' M&S "has increased pressure on its competitors to demonstrate their own efforts", Citigroup added.

"M&S has at least a six-month lead on the four largest UK food retailers," the firm 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."

Citigroup said there is potential for this approach to be extended to other food categories.

It added: "We believe the company's efforts to inform consumers on key topics of ethical procurement and production are contributing to the ongoing sales recovery and will underpin the brand's performance going forward."

M&S launched its campaign in January with window displays in 420 stores providing information on ethically traded products, health and quality.

Stores began stocking T-shirts and socks made entirely from cotton approved by the Fairtrade Foundation - which ensures farmers in Mali, India and Senegal get a fair price for products - in March.

This proved so successful it was followed by the sale of vintage style jeans and underwear made from Fairtrade cotton.

M&S pledged to remove all hydrogenated fats from food by the end of the year and is cutting down on salt and removing artificial flavouring from its ranges. It also replaced all 38 lines in its tea and coffee ranges with Fairtrade alternatives.

A spokeswoman for M&S said: "'Look behind the label' is a very important campaign for us."

Citigroup said Waitrose, owned by John Lewis, could present some challenges in the ethical field but has significantly smaller floor space and lower exposure to the convenience sector.

Earlier this month, a survey from TNS found M&S was rated the most ethical retailer in the UK.