  That the speed of light in free space is constant is a cornerstone of modern
physics. However, light beams have finite transverse size, which leads to a
modification of their wavevectors resulting in a change to their phase and
group velocities. We study the group velocity of single photons by measuring a
change in their arrival time that results from changing the beam's transverse
spatial structure. Using time-correlated photon pairs we show a reduction of
the group velocity of photons in both a Bessel beam and photons in a focused
Gaussian beam. In both cases, the delay is several microns over a propagation
distance of the order of 1 m. Our work highlights that, even in free space, the
invariance of the speed of light only applies to plane waves. Introducing
spatial structure to an optical beam, even for a single photon, reduces the
group velocity of the light by a readily measurable amount.
