  Photoproduction events which have two or more jets have been studied in the
$W_{\gamma p}$ range 135~GeV $< W_{\gamma p} <$ 280~GeV with the ZEUS detector
at HERA. A class of events is observed with little hadronic activity between
the jets. The jets are separated by pseudorapidity intervals ($\Delta\eta$) of
up to four units and have transverse energies greater than 6~GeV. A gap is
defined as the absence between the jets of particles with transverse energy
greater than 300~MeV. The fraction of events containing a gap is measured as a
function of $\Delta\eta$. It decreases exponentially as expected for processes
in which colour is exchanged between the jets, up to a value of $\Delta\eta
\sim 3$, then reaches a constant value of about 0.1. The excess above the
exponential fall-off can be interpreted as evidence for hard diffractive
scattering via a strongly interacting colour singlet object.
