  The production of dijets with a tagged forward proton is measured at HERA.
The data were recorded with the H1 detector at DESY in the years 2006-2007.
Events with a leading proton are detected using the very forward proton
spectrometer of the H1 detector. Two jets are selected with transverse momenta
transverse momenta in the hadronic-centre-of-mass larger than 4 and 5.5GeV,
respectively. The analysis is performed both in the regime of deep-inelastic
scattering (DIS), with momentum transfer $Q{^2} > 4$ GeV${^2}$ and for
photoproduction ({\gamma}p), with $Q{^2} < 2$ GeV${^2}$. Cross sections are
measured single-differentially in various kinematic quantities. For DIS, the
data are found to be in good agreement with NLO QCD calculations based on
diffractive parton densities determined from inclusive diffractive cross
section measurements. For {\gamma}p, the cross sections are found to be
overestimated by approximately a factor of two.
