A new survey has ranked Asda the "least green" of the UK's major supermarkets, with its green rating slipping since last year.

According to an annual mystery shopping exercise conducted by Consumer Focus, Asda achieved a D grade on its sustainability score card, dropping from a C in 2007.

The survey looked at a range of issues from environmentally friendly practices in-store - such as closed freezer doors or recycling facilities - to the availability of sustainable, seasonal or local products.

"Shopping green should not be hard. Our survey shows that when a grocer has the will to respond to consumer pressure on green issues, this works through to the shop floor. Saying you have a sustainable fish sourcing policy is not enough - consumers must be able to find sustainable fish on the shelves," Lucy Yates, sustainability expert at Consumer Focus, insisted.

Marks and Spencer and Sainsbury's were the highest scoring supermarkets - both achieving an A grade - followed by Waitrose who scored a B.

However, Asda has refuted the survey's findings. A spokesperson for the group told just-food that the report was "inaccurate" and "misleading".

"This report isn't worth the recycled paper it's written on," the spokesperson insisted.