  We present Johnson-Kron-Cousins BVRI photometry of 228 candidate
spectrophotometric standard stars for the external (absolute) flux calibration
of Gaia data. The data were gathered as part of a ten-year observing campaign
with the goal of building the external grid of flux standards for Gaia and we
obtained absolute photometry, relative photometry for constancy monitoring, and
spectrophotometry. Preliminary releases of the flux tables were used to
calibrate the first two Gaia releases. This paper focuses on the imaging frames
observed in good sky conditions (about 9100). The photometry will be used to
validate the ground-based flux tables of the Gaia spectrophotometric standard
stars and to correct the spectra obtained in non-perfectly photometric
observing conditions for small zeropoint variations. The absolute photometry
presented here is tied to the Landolt standard stars system to $\simeq$1 per
cent or better, depending on the photometric band. Extensive comparisons with
various literature sources show an overall $\simeq$1 per cent agreement, which
appears to be the current limit in the accuracy of flux calibrations across
various samples and techniques in the literature. The Gaia photometric
precision is presently of the order of 0.1 per cent or better, thus various
ideas for the improvement of photometric calibration accuracy are discussed.

Solar and Stellar Astrophysics