National self-image can be a deceptive thing. Economists tell us that Britain is in a healthy state compared with the rest of Europe. And we are used to hearing about the vibrancy of our cultural life. Yet when it comes to our physical health, the official data released yesterday shows that Britain is actually in a rather poor condition. As the head of the Food Standards Agency points out, we are now the sick man of Europe.

The latest Health Profile of England also reveals a stark geographical divide. People in large parts of the north of England have a lower life expectancy than those in the south. And although Scotland is not included in the survey, it is no secret that health levels get worse the further north one travels.

This discrepancy cannot be attributed solely to income. Places with the worst health are by no means always the poorest - and Britons are, per head, just as wealthy as our healthier continental peers. The real difference here is lifestyle. Habits such as smoking and drinking play their part in making some parts of the nation more prone to illness than others. So does the relative lack of exercise we take. But much of the problem lies in diet. Large parts of Britain, for a variety of reasons, eat poor quality and unhealthy foods - something that leads to obesity, illness and, ultimately, an earlier death.

The Prime Minister was right yesterday to stress the importance of establishing the principle of "preventative" health care here if we are to see any improvement. We cannot continue to regard the NHS as a "national illness service". The Department of Health predicts that 13 million people in England will be obese by 2010. With obesity-related illnesses already costing the nation some £3.5bn a year, this could eventually bankrupt the NHS. The success of the Cuban health service shows that when doctors focus not just on a patient's ailments, but on their general lifestyle, the results can be astonishing.

But the real test for the Government lies in whether it can persuade Britons to eat more healthily. There has been some success. The quality of school meals has shown an improvement after the introduction of new guidelines. But the objectives laid out in the Government's health White Paper of two years ago look distant. Many deprived areas are still fresh-food deserts. And the Government's proposed "traffic-light" system for food labelling has been rejected by the food industry.

The Government cannot force people to eat more healthily, but it can do a lot more to encourage it. And a good deal more than our national self-image is riding on the success of such efforts.