The variation of the muon charge ratio at sea level for near-vertical incidence within the momentum range 4 to 2000 GeV/c has been determined using the Nottingham cosmic-ray spectrometer. These results have been combined with the data reported by other workers since 1970 to provide accurate information over this momentum range. A comparison of the theoretical variation of the charge ratio obtained using the isobar model to describe particle interactions shows that good agreement can be obtained between experiment and theory.