  Water masers are found in dense molecular clouds closely associated with
supermassive black holes in the centres of active galaxies. Based upon the
understanding of the local water maser luminosity function, it was expected
that masers at intermediate and high redshifts would be extremely rare, but
galaxies at redshifts z > 2 might be quite different from those found locally,
not least because of more frequent mergers and interaction events. Using
gravitational lensing as a tool to enable us to search higher redshifts than
would otherwise be possible, we have embarked on a survey of lensed galaxies,
looking for masers. Here we report the discovery of a water maser at redshift
2.64 in the dust- and gas-rich gravitationally lensed type 1 quasar MG
J0414+0534, which, with an isotropic luminosity of 10,000 L_solar, is twice as
luminous as the most powerful local water maser, and half that of the most
distant maser previously known. Using the locally-determined luminosity
function, the probability of finding a maser this luminous associated with any
single active galaxy is 10^{-6}. The fact that we saw such a maser in the first
galaxy we observed must mean that the volume densities and luminosities of
masers are higher at redshift 2.64.
