Strategies to combat obesity, waste and climate change are doomed to fail unless government can work with big supermarket chains, claims a leading policy think tank.

In Green, Healthy and Fair, a report published today (Saturday), the Sustainable Development Commission said too many supermarket practices were "unhealthy, unjust and unsustainable".

The SDC singled out the "aggressive promotion" of processed foods, which it claimed made fresh produce appear expensive and unappealing. In this environment, the 5-a-day message could never succeed. Also of concern were over-packaging and non-recyclable packaging, which the SDC said fuelled waste.

Tim Lang, professor of food policy at London's City University and one of 19 commissioners at the SDC, said the solution to many of these problems was for the government to harness the power of big retailers.

"Government cannot resolve the problems of obesity, waste or climate change alone," he said. "Given the enormous influence wielded by supermarkets, working with them effectively is essential."

But the SDC warned that conflicting policies from government departments were making it hard for supermarkets to respond to government-led initiatives.

There were 19 Whitehall departments with a total of almost 100 policy responsibilities related to food, the report said. An example of conflict was the Department of Health's advice to eat more fish, which cut across attempts to preserve endangered fish stocks - although the FSA has now said it will review its advice on fish consumption.

"With public scrutiny of retailers' behaviour increasing, many supermarkets are keen to work with government to develop a green, healthy and fair food system," said Lang. "In fact, they are often frustrated by the lack of clarity of long-term strategy."

The SDC, the government's independent advisory body on sustainability issues, which reports directly to the prime minister, also recommended the food sector agreed on a single mandatory front-of-pack nutrient labelling system.