Inner city boroughs may overtake former industrial towns as the unhealthiest places in Britain, researchers say.
Market analysts CACI and TNS looked at smoking and disease rates and dietary habits to plot the least healthy places in England, Wales and Scotland.
The report said towns such as Merthyr Tydfil in Wales were the least healthy, with poor diets and smoking causing heart disease and other problems.
But it said inner cities are storing up problems, with bad habits in the young.
The report said the health service must prepare for a "timebomb" caused by widespread unfit and unhealthy lifestyles.
It comes after a British Medical Journal article warned last month that obesity - already responsible for 9% of the NHS budget - could bankrupt the NHS in the future.
The study said eight of the 10 most unhealthy places with high levels of serious illness, such as heart disease and obesity, were in south Wales.
But they also warned inner city areas of London, Nottingham, Hull and Manchester, which commonly had young populations with unhealthy habits and high levels of single-parent families, were likely to face serious problems in the future.
Researcher Ian Thurman said: "The danger for the NHS is that this group could be presenting itself with poor health at a relatively young age."
He said it was "inevitable" that these deprived urban areas would replace south Wales communities as the most unhealthy places as the Welsh towns got over the health legacy of heavy industry.
"It looks like we have got a set of attitudes to food that look fairly entrenched in terms of fast food and high levels of fat."
Mr Thurman added: "The NHS is already overburdened, but this is nothing compared to the timebomb that is set to explode if people don't make major changes to their current lifestyles.
"This analysis goes beyond simple measurement of existing health patterns and identifies where early intervention can be most effective in reducing future illnesses.
"This means health care and support can be developed for small communities where ill-health prevention activities can be accurately targeted."
The Isles of Scilly, Eden in Cumbria, Orkney Islands and parts of London were said to be the healthiest.
But Professor Danny Dorling, a Sheffield University health inequalities expert, said it was not necessarily certain inner city areas would become more unhealthy in the future.
"What is common with many of these areas is that their populations have a large proportion of young people, especially students, who will develop healthier lifestyles as they get older and probably move out of the areas.
"The pattern of healthy and unhealthy areas has remained largely the same for the last 150 years or so. I don't think it will change dramatically."
The Welsh Assembly Government said it was encouraging people to take better care of themselves, but the country had a "history of ill health and chronic conditions due mainly to its industrial past".
"We are encouraging children to take care of themselves, teaching valuable health lessons that they can carry through to adulthood through initiatives in schools such as free healthy breakfasts, fruit tuck shops and water coolers."