The society has launched a new website and rolled out TV and press ads claiming it sells "more Fairtrade in more stores than anyone else". The push is backed by promotions on Fairtrade lines such as coffee, orange juice and Champagne chocolate truffles.
"As well as reinforcing our Fairtrade credentials and our position as the leading supermarket supporter of Fairtrade, this campaign will raise awareness of the wide range of products on offer at Co-op and Somerfield stores," said Brad Hill, Fairtrade strategy development manager for The Co-op Group.
Sainsbury's was unimpressed by The Co-op Group's claims. "Their claim will be based on having more stores than Sainsbury's but we are the world's biggest retailer of Fairtrade products, with sales of £218m," a spokesman said. "This means £1 in every £4 spent on Fairtrade in the UK is spent at Sainsbury's, so we are the leading Fairtrade retailer by some way."
A spokeswoman for The Co-op Group responded: "Our track record on pioneering Fairtrade speaks for itself. We've always said it's good for everyone the more retailers get behind Fairtrade; for consumers who get more access to these products; and for growers in developing countries who reap the benefits."
Both retailers have pushed Fairtrade hard. The Co-op Group is the only retailer to have converted all block chocolate and own-label hot beverages to Fairtrade and has the largest accredited own-label wine range. Sainsbury's converted its entire banana range to Fairtrade in 2006 and sells more than 800 Fairtrade products in its stores.
The Fairtrade Foundation said it was "very refreshing to see supermarkets striving to lead on ethics rather than price.
"Both can take credit for leading on Fairtrade," said policy and communications director Barbara Crowther.