Abstract Background  The prevalence of Type 2 diabetes is increasing worldwide; predictions suggest that the disease will reach epidemic proportions this century. This study aims to estimate the extent of the increase in prevalence of diagnosed Type 2 diabetes in British men between 1978 and 2005. Methods  A representative cohort of 7722 British men aged 40–59 years at entry in 1978–1980 were selected from general practices in 24 British towns. Seven sequential questionnaire surveys were carried out between 1978 and 2005, recording recall of a doctor diagnosis of diabetes at each time point. Logistic regression models with generalized estimating equations were fitted to provide age‐adjusted estimates of the calendar year increases in odds of Type 2 diabetes, both overall and for consecutive periods, each of approximately 5 years. Results  The crude prevalence of Type 2 diabetes increased from 1.2% in 1978–1980 to 12.1% in 2005. The age‐adjusted average annual increase in Type 2 diabetes prevalence for the 27‐year study period was 7.0% [95% confidence interval (CI) 5.4%, 8.6%]. However, the age‐adjusted annual rate of increase increased over time, from 4.3% (95% CI 0.4%, 8.2%) between 1979 and 1984 to 11.8% (95% CI 8.4%, 15.4%) between 2003 and 2005; P (trend) = 0.01. The highest annual increases occurred in subjects with higher mean body mass index levels and in towns in Scotland. Conclusions  The prevalence of Type 2 diabetes has risen substantially in Britain during the last three decades; the recent rate of increase has been almost three times greater than that in the early 1980s.