  We report the discovery of J0121+0025, an extremely luminous and young
star-forming galaxy (M_UV = -24.11, log[L_Lya / erg s^-1] = 43.8) at z = 3.244
showing copious Lyman continuum (LyC) leakage (f_esc,abs ~ 40%). High
signal-to-noise ratio rest-frame UV spectroscopy with the Gran Telescopio
Canarias reveals a high significance (7.9 sigma) emission below the Lyman limit
(< 912A), with a flux density level f_900A = 0.78 +/- 0.10 uJy, and strong
P-Cygni in wind lines of OVI 1033A, NV 1240A and CIV 1550A that are indicative
of a young age of the starburst (<10 Myr). The spectrum is rich in stellar
photospheric features, for which a significant contribution of an AGN at these
wavelengths is ruled out. Low-ionization ISM absorption lines are also
detected, but are weak (EW0 ~ 1A) and show large residual intensities,
suggesting a clumpy geometry of the gas with a non-unity covering fraction or a
highly ionized ISM. The contribution of a foreground and AGN contamination to
the LyC signal is unlikely. Deep optical to Spitzer/IRAC 4.5um imaging show
that the spectral energy distribution of J0121+0025 is dominated by the
emission of the young starburst, with log(M*/Msun) = 9.9 +/- 0.1 and SFR = 981
+/- 232 Msun yr^-1. J0121+0025 is the most powerful LyC emitter known among the
star-forming galaxy population. The discovery of such luminous and young
starburst leaking LyC radiation suggests that a significant fraction of LyC
photons can escape in sources with a wide range of UV luminosities and are not
restricted to the faintest ones as previously thought. These findings might
shed further light on the role of luminous starbursts to the cosmic
reionization.
