For a man who is internationally renowned as an authority on food policy and safety, Tim Lang's office is somewhat pokey and, dare one say it, unhygienic. Every stray piece of carpet and desk is covered with books and paper and the ceiling tiles are brown with damp and mould. It's anyone's guess whether Lang and I are the only living creatures in the room.

"I don't eat off the ceiling," he says defiantly. "In any case, I love this office; it's the only office I've ever had to myself in my entire working life. I can play whatever music I like and no one can complain."

It's hard to believe that it wasn't until two years ago, at 54, when he and his research team moved to City University from Thames Valley University, that Lang finally acquired a room of his own. This may speak eloquently of the financial hardships of British higher education, but it probably tells us more about this country's relationship with food.

For most of the past 50 to 60 years, successive governments - and the population with them - have taken a productionist view of agriculture. After the country came close to starvation during the first and second world wars, the postwar Labour government determined it would not happen again. Subsidies were offered to farmers, food was produced in industrial quantities and a blind eye turned to the injustices in the system and the harm done to public health and the environment.

By the late 1980s, the first cracks emerged with a series of food scares, and by the time of the BSE and foot and mouth crises, the writing was clearly on the wall: agribusiness had fatal flaws. These fault lines would have appeared regardless, but it was Lang and others on the Food Commission who exposed many of the food scandals and predicted the rest. Which made him public enemy number one for the multinational food producers and a maverick irritant to government.

These days, though, Lang has stepped up a gear, from the oddball sheep farmer turned academic to the mainstream. His passion has remained constant, though, and his incorporation into the establishment - he's served on umpteen committees and frequently advises the government and the World Health Organisation - is partly a matter of expediency: it's far better to have your critics on the inside than kicking you from the outside.

But it's also a recognition that Lang has largely been proved right. In fact, his views are now so widely accepted that last week, his latest book, The Food Atlas, won the prestigious Andre Simon award for food writing. So something has to change.

The questions now are what and how much. "The Labour government has taken some important steps," Lang says. "The creation of the independent Food Standards Agency [FSA] sent a powerful message to business, and I broke open a bottle of champagne when the Ministry for Agriculture, Food and Fisheries [Maff] was disbanded.

"Maff had been completely captured by the large food producers and was incapable of implementing the necessary changes. Its incorporation into the Department for Environment, Food and Rural Affairs [Defra] showed a recognition of the different relationships within the food chain."

Lang is the first to admit that changing the architecture of government control of the food industry must be followed up with action. And he has his doubts about just how committed to the process the government is.

Genetically modified foods might be a case in point. Despite widespread public doubts about the long-term effects of GM, the chairman of the FSA, Sir John Krebs, has turned out to be one of GM's strongest advocates. And partly on his advice, the government last week gave the not-unexpected go-ahead to the cultivation of GM maize.

"The government has always had a love affair with biotech companies," Lang says, "and it's been clear since the mid-80s that these companies targeted the UK as the backdoor into Europe."

But he also suspects that the government is not as enthusiastic about GM as it seems. "Margaret Beckett's announcement came with an unpleasant sting in the tail," he explains. "While giving the go-ahead for GM maize, she also implied there might be a requirement for liability and traceability.

"This opens the supply chain - from producer to retailer - to financial pressures to which they are not accustomed. No one knows just who would and would not be liable, and it would be a foolish company who took the government's statement as an unequivocal green light."

Lang may be closely linked to the environmental lobbies, but he insists he is not viscerally or intellectually opposed to GM. "It's just that I remain unconvinced," he says. "Agriculture has been a 10,000-year process and many changes have taken places over decades or centuries. GM has gone from development to application in just 10 years and that's not enough time to understand its effects.

"I also don't believe that GM is going to revolutionise the food production system. The biotech companies developed an application and then went looking for areas in which to use it; they didn't study what the food industry needed and try to develop an appropriate response. Will GM foods make any difference to the unequal distribution of food, where some people over-consume while others starve? Will they reduce levels of obesity? Emphatically not."

If GM gets Lang going, it's public health and obesity that send him into overdrive. He spits blood at the Department of Health's inertia. "Did you know it was the last government agency to respond to the Curry Commission on the future of farming and food?" he says. "We spend a fortune on healthcare and yet the relevant department can't even be bothered to think about the causes of the nation's health problems.

"Diabetes costs us £1.2bn a year; cardiovascular disease costs £9bn: diet-induced cancers come to another £4bn. The real cost of our cheap food policy is a healthcare system we just can't afford."

Lang has offered his own solutions to the never-ending stream of denaturised, artery-busting cheap foods that make up much of the developed and, increasingly, the undeveloped world's diet. He has suggested taxes on advertising unhealthy foods and on sugar-soaked drinks, but he acknowledges that what is really required is a concerted, long-term effort both from our own and foreign governments - something that is lacking at present.

Lang is on Defra's horizon panel to assess the likely food risks over the next 30 years and is all too aware his challenge will be to get the government to commit to its findings. But he feels the stakes are too high to give up.

"It's a global problem," he points out. "Greece, the cradle of the Mediterranean diet, has one of the fastest growing rates of obesity. At the present rate in the UK, it is estimated it will take until 2040 before everyone eats the minimum requirement of five portions of fruit and vegetable per day. In the meantime, we are consigning two generations to an early grave."

The CV

Name Tim Lang

Age 56

Job Professor of food policy, City University; consultant to World Health Organisation; Defra horizon scanning team

Before that Professor of food policy, Thames Valley University: director, London Food Commission; special adviser to four House of Commons committee enquiries

Books Food Wars (with M Heasman) 2004; Atlas of Food (with E Millstone) 2003

Likes breakfast, birdwatching

Dislikes eggs, social injustice

Nearly married, two stepchildren