Marks and Spencer is streets ahead of its rivals in terms of its ethical positioning - but faces stiff competition from newcomer Whole Foods Market, say analysts.

Citigroup said research by M&S had indicated that the retailer's Look Behind The Label campaign was its most successful ever in terms of brand perception.

The retailer had "at least a six-month lead" on the four largest chains in terms of ethics, said Citigroup, adding: "We expect it to use this advantage to introduce new areas of responsibility, particularly in areas where its competitors will not be able to follow. There is the potential for this approach to be extended. Could we see 'all free-range meat' or 'all organic vegetables'? "Such an approach could appeal to the time-poor shopper who could prefer M&S as it would take away the need to search shelves for products that meet their ethical standards."

But the report warns that Whole Foods Market, which opens its first store in Kensington next year, will pose a fresh challenge.