Excess weight is associated with several chronic conditions and increased total healthcare costs. However, little is understood about the impact of excess weight on the use and costs of primary healthcare services in the United Kingdom. Using individual participant data from a large cohort of women aged above 50 years in England (the Million Women Study), linked to routinely collected primary care data from the Clinical Practice Research Datalink, we describe and quantify the relationships between BMI and the number and costs of primary care consultations, tests, and prescriptions issued. 69,440 women who provided information on height and weight, had a BMI of ≥18.5kg/m2, and had no previous cancer at recruitment, were followed for an average of 6.0 years from April 2006 (recruitment 1996 to 2001). Annual rates and costs of consultations (7.0 items, 99% CI 6.8-7.1; £288, 280-295) and prescription items issued (27.0 items, 26.0-27.9; £227, 216 to 237) were lowest for women with a BMI of 20-22.5kg/m2. Every 2 kg/m2 increase in BMI beyond 20 kg/m2 was associated with 5.2% (4.8-5.6) and 9.9% (9.2-10.6) increase in annual consultation and prescription costs, respectively. Annual rates and costs of tests were similar for women at different BMIs. Among all women aged 55-79 years in England, 11% (£229million/£2.2billion) and 20% (£384million / £1.9billion) of all consultation and prescription costs, respectively, were attributable to overweight and obesity. 27% of the annual prescription costs attributable to overweight and obesity were for diabetes medications, 19% for cardiovascular medications, and 13% for analgesics. These results provide reliable estimates of the primary care costs of excess weight and emphasise the need for investment to reduce the prevalence of obesity and the burden on primary care.