Background There are more people registered with a general practice in England than are estimated to be resident in the country. The reasons behind this are not fully understood. We investigated the levels of over-registration (or under-registration) in English primary care, their regional variability and their association with population and geographical characteristics.

Methods This was a cross-sectional study using mid-year population estimates for 2014 and general practice populations for the same year. We calculated levels of patient registration with English primary care, in relation to census-derived population estimates, at various geographical levels of interest: regions, clinical commissioning groups and lower super output areas (LSOAs, 2011 census derived geographical areas of 1500 people on average). We used linear regressions to investigate the relationship between levels of registration and area deprivation, urbanicity, ethnicity, age, sex and mean distance to practice.

Results The total over-registration rate for England was 3.9% (2 097 101 people) but there was wide regional variability. London had significantly higher levels of over-registration (6.0% and 515 063 people) than other areas in England. Higher levels of over-registration at the LSOA level were associated with greater proportions of non-White British residents, women, elderly people and higher levels of social deprivation.

Conclusion Our findings indicate that high mobility and health need may be the underlying causes of over-registrations. The regional variation in over-registration, with London being an outlier, points towards potential inequalities in resourcing of primary care and the ability of the National Health Service to adequately match funding to population need.