It is not Britain's addiction to junk food that is chiefly to blame for our obesity crisis, but our love affair with the car, a report will claim this week.

Researchers have found that when households take possession of a car the time they spend walking falls from an average of nearly two hours to less than one hour a week.

Experts are still arguing over what is generating Britain's growing obesity crisis - more food or less exercise? The report from the Institute for European Environmental Policy claims to show that the main cause is falling activity levels - and the biggest culprit is not less time spent at the gym or playing sport, but simply walking around less.

'We're not saying that a decline in walking is totally responsible for the rise in obesity, but the evidence points to the fact that it's a decline in physical activity that is the major determinant, and it's a replacement of walking by car use which is the main determinant [of that],' said Adrian Davis, an expert in health and transport policy and one of the authors of the report.

The researchers found that since the mid-Seventies the proportion of households without a car has halved, from 41 per cent to 19 per cent. At the same time the average time an adult spends walking has fallen from 87 hours to 67 hours a year, while time spent driving has risen from 91 to 151 hours. Based on these figures, the report calculates that the average adult burns 26 fewer calories a day - leading to an average weight gain of 2lb 11oz each year.

The study is part of an attempt to link health issues with the environmental pollution caused by motoring, and to encourage ministers to tackle both by promoting more walking. The institute is calling for towns and cities to be made more pedestrian-friendly, for example by making pavements wider and improving street lighting.

The study will further fuel the debate over the causes of obesity, which is linked to heart disease, cancer and diabetes. It cites previous studies claiming that British people are not consuming many more calories than a generation ago, so lower activity levels must be to blame.

However, Tim Marsh, associate director of the National Heart Forum, said other evidence suggested that people were eating more - by snacking, rather than at mealtimes - but were 'under reporting' how much they consumed. This argument was boosted last week by a study from the Medical Research Council, which claimed that fatter people were more likely to underestimate their consumption. 'You have to do an awful lot of walking to burn off the calories,' added Marsh.

Healthy steps

An hour's brisk walking will burn off ...

0.5 McDonald's Quarter Pounders with Cheese

1.2 Snickers chocolate bars

1.5 cans of Coca-Cola

4.3 glasses of wine

Assumes an adult weight of 10.5 stones.