Eight in 10 British workers are overweight or living with long-term illnesses that limit their productivity, according to early findings of a 25-year study of people's wellbeing.

Poor health and obesity is costing the economy at least £21.5bn a year and will prove a severe drag on any recovery from the recession, the study suggests.

Workers who are both overweight and have three or more health conditions – more than 10% of the total – are taking over three weeks' sick leave every year.

The findings come ahead of publication, due in October, of an independent review of sickness absence, commissioned by the government. That review is expected to warn that growing numbers of workers are living with long-term conditions and need greater support to do so.

According to the wellbeing study, based on initial telephone interviews with almost 4,000 full-time workers, only 20% are not overweight and have no health conditions, ranging from high blood pressure to cancer. Another 20% have a weight problem but no health conditions. But six in 10 have at least one condition and 16%, almost one in six, have three or more.

The study, by pollsters Gallup, is a 25-year collaboration with Healthways, an international wellbeing consultancy, in the UK, Germany and the US. Surveys in Britain began earlier this year and have so far involved almost 9,000 people in and out of employment.

Dan Witters, Gallup principal and research director of the study, said: "Only one in five British full-time workers are in optimal health. Because of this, they miss an estimated 103m days at work a year."

Ben Leedle, Healthways president and chief executive, said: "The implication of the chronic disease burden of the UK's workforce is at alarming levels."

Gallup says the findings are a conservative estimate of productivity loss because they do not include part-time workers and take no account of "presenteeism" – people turning up to work when they are ill.

Conditions counted in the survey include recurring back or knee pain as well as diagnosed depression. However, they do not include stress or anxiety, which are among the fastest growing causes of sickness absence.

The international study has found that while obesity and most health conditions are more common in the US, Britain has the highest rate of asthma and a rate of depression 50% higher than in Germany.