  We report the detection of the Lyman continuum (LyC) radiation of the compact
star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins
Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a
redshift of z=0.3690, is characterized by a high emission-line flux ratio
O32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation
fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in
low-redshift SFGs and one of the highest values found in galaxies at any
redshift. The narrow double-peaked Lya emission line is detected in the
spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s,
one of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya).
Comparing the extinction-corrected Lya/Hb flux ratio with the case B value we
find fesc(Lya) = 98 per cent. Our observations, combined with previous
detections in the literature, reveal an increase of O32 with increasing
fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The
surface brightness profile derived from the COS acquisition image reveals a
bright star-forming region in the centre and an exponential disc in the
outskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to
other known low-redshift LyC leakers, is characterized by the lowest
metallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a
similar star formation rate SFR=18.9 Msun/yr and a high specific SFR of
1.2x10^-7 yr^-1.
