The stark social class divide in health is widening as better-off people increasingly shun damaging habits such as smoking and eating badly but poorer people do not, authoritative new research reveals.

The number of people in England who risk their health by engaging in "multiple lifestyle behaviours" such as smoking, drinking too much and physical inactivity is falling, according to a study by the influential King's Fund health thinktank, shown to the Guardian.

But it is middle-class people who are driving the trend, while the most disadvantaged remain stuck in a cycle of risky behaviour which increases their chances of developing a range of serious illnesses and will put "unavoidable pressure" on an already hard-pressed NHS, it says.

The study covered 2003-08, a period in which the Labour government embarked on a massive campaign to persuade people to live healthier lives.

The researchers found that people with no educational qualifications were more than five times as likely as those with degrees to engage in four key damaging behaviours in 2008, compared with three times as likely in 2003.

The findings have cast doubt on the prospect of the health secretary, Andrew Lansley, fulfilling his pledge to "improve the health of the poorest fastest" in order to reduce glaring health inequalities. England's richest citizens live seven years longer on average than their poorer neighbours. The report also reopens the debate over how to encourage poorer people to adopt healthier habits.

The increasing divide was identified by a research team led by David Buck, a senior fellow at the King's Fund who was head of health inequalities at the Department of Health until 2010. The team analysed data from the NHS's health survey for England covering four behaviours which are closely linked to disease and early death: smoking, excess alcohol use, poor diet and sedentary lifestyles. Between them, these four habits are estimated to account for almost half the burden of ill-health in developed countries.

The researchers found that the number of people engaging in three or four of these risky behaviours fell from 33% in 2003 to 25% in 2008 – a "significant" and, until now, unnoticed improvement that will benefit public health.

But when they looked at what types of people were quitting smoking or drinking less, for example, they found that "these reductions have been seen mainly among those in higher socioeconomic and educational groups".

"The health of the overall population will improve as a result of the decline in these behaviours, but the poorest and those with least education will benefit least, leading to widening health inequalities and unavoidable pressure on the NHS," the study says.

Those from poorer backgrounds or with less education are more likely than others to develop long-term conditions such as cancer, diabetes and cardiovascular disease earlier and to experience them more severely, Buck said. "As well as this being a public health problem, this does also store up problems for the NHS in future, as people with these conditions will be a drain on the NHS at a time when its resources are going to be very constrained," he explained.

The authors found that the better off someone is, the more likely they are to have begun living a healthier life during 2003-08, a period when the government funded a host of often dramatic awareness campaigns aimed at changing attitudes to smoking, reducing harmful drinking and tackling obesity.

"The [widening] inequality gap is due to the improvement in those at the top, and, to a lesser degree, those in the middle, not because those at the bottom have got worse per se. They're stuck in a rut," Buck said.

The study warns that there is no room for complacency because about 70% of adults in England – and more among poorer people – still engage in two of the four habits.

"This is worrying as, if the current government is to be taken at its word, the key test of its health policy is whether it is seen to be 'improving the health of the poorest, fastest", the authors add. "It will be judged on whether its policies on lifestyle risk really do drive change that helps us all, but particularly the poorest in society, to move down the risk ladder from four unhealthy behaviours to none."

Greater use of health trainers, community health champions, an expansion of the "every contact counts" idea suggested by the NHS Future Forum – in which every health professional talks to patients about their lifestyle every time they see them -- and linking the income of both hospitals and GPs to reducing the number of people displaying three or four of the behaviours may help reduce the gap, the authors suggest.

"There's a huge challenge here to overcome this very stark divide, but it's not an impossible one", said Professor Lindsey Davies, president of the Faculty of Public Health. But she voiced concern that the new NHS commissioning board, which will run the service from next April, had no public health experts on its board. Public health professionals' knowledge of what works to wean people off these clusters of unhealthy behaviour is also "in the Dark Ages" and needs to improve, she added.

Gabriel Scally, a professor of public health who was one of the NHS's most senior public health doctors until April, said: "These findings should come as no surprise because we know that the factors lying behind thinks like smoking or poor diet, such as poverty or poor housing, cluster together for some people and effect the way people live their lives, just as these unhealthy behaviours are also often found in clusters.

"There's a real danger that we will see a widening of health inequalities because of the government's refusal to set targets for improving health across England. It's always been the case that middle class people have adopted healthier lifestyles faster, such as giving up smoking. But we are seeing a stark and growing class divide which can only be increased by the effects of the recession, unemployment, falling incomes and benefit cuts."

The Department of Health said: "We are working hard to tackle health inequalities. From next year, local authorities will receive a specific public health budget for the first time, targeted at the areas that need it most. Additionally, the Health Act has given the NHS its first ever duties concerning the need to reduce health inequalities."