Abstract : A novel algorithm to reconstruct neutrino-induced particle showers within the ANTARES neutrino telescope is presented. The method achieves a median angular resolution of $6^\circ $ for shower energies below 100 TeV. Applying this algorithm to 6 years of data taken with the ANTARES detector, 8 events with reconstructed shower energies above 10 TeV are observed. This is consistent with the expectation of about 5 events from atmospheric backgrounds, but also compatible with diffuse astrophysical flux measurements by the IceCube collaboration, from which 2–4 additional events are expected. A $90\%$ C.L. upper limit on the diffuse astrophysical neutrino flux with a value per neutrino flavour of $\text {E}^2\cdot \Phi ^{90\%} = 4.9 \cdot 10^{-8}\,\mathrm {GeV} \cdot \mathrm {cm^{-2} \cdot s^{-1}\cdot sr^{-1}}$ is set, applicable to the energy range from 23 TeV to 7.8 PeV, assuming an unbroken $\text {E}^{-2}$ spectrum and neutrino flavour equipartition at Earth.
