  High-significance evidences of the existence of a high-energy diffuse flux of
cosmic neutrinos have emerged in the last decade from several observations by
the IceCube Collaboration. The ANTARES neutrino telescope took data for 15
years in the Mediterranean Sea, from 2007 to 2022, and collected a high-purity
all-flavour neutrino sample. The search for a diffuse cosmic neutrino signal
using this dataset is presented in this article. This final analysis did not
provide a statistically significant observation of the cosmic diffuse flux:
this is converted into limits on the properties of the cosmic neutrino
spectrum. In particular, given the sensitivity of the ANTARES neutrino
telescope between 1 and 50 TeV, constraints on single-power-law hypotheses are
derived for the cosmic diffuse flux below 20 TeV.
