Context: The variable pre-white dwarf PG 1159 stars (GW Vir) are g-mode non-radial pulsators. Asteroseismology puts strong constraints on their global parameters and internal structure. PG 0122+200 defines the red edge of the instability strip and its evolutionary timescale is predicted to be dominated by neutrino emission. Its study offers the opportunity to better understand the instability mechanism and to validate the physics of the neutrino production in dense plasma. Aims: To achieve such a goal requires determining precisely its fundamental parameters. This is the goal of this paper. Methods: We present new multi-site photometric observations obtained in 2001 and 2002. Together with previous data, they allow us to detect 23 frequencies, composed of 7 triplets and 2 single frequencies, which are used to constrain its internal structure and derive its fundamental parameters. Results: All the observed frequencies correspond to ℓ=1 g-modes. The period distribution shows a signature of mode trapping from which we constrain the He-rich envelope mass fraction to be -6.0≤ log(qy) ≤ -5.3. The comparison of the mode trapping amplitudes among GW Vir stars suggests that the mass-loss efficiency must decrease significantly below T_eff≤ 140 kK. We measure an average period spacing of 22.9 s from which we derive a mass of 0.59±0.02 M&sun;. From the triplets we measure a mean rotational splitting of 3.74 μHz and a rotational period of 1.55 days. We derive an upper limit to the magnetic field of B≤4×103 G. The luminosity (log L/L&sun; = 1.3±0.5) and the distance (D = 0.7^+1.0_-0.4 kpc) are only weakly constrained due to the large uncertainty on the spectroscopically derived surface gravity and the absence of a measured parallax. Conclusions: From the asteroseismic mass, the ratio of the neutrino luminosity on the photon luminosity is 1.6±0.2 confirming that the PG 0122+200 evolutionary time scale should be dominated by neutrino cooling. A measurement of dot{P} for the largest amplitude untrapped modes should verify this prediction. Based on data obtained at the Haute-Provence Observatory, INSU/CNRS, France; the Nordic Optical Telescope, La Palma, Spain; the Xinglong station of National Astronomical Observatories, China; the Bohyunsan Observatory, South-Korea; the Gunma Astrophysical Observatory, Japan; the Teide Observatory, Tenerife, Spain; the Calar Alto Observatory, Spain; the Kitt Peak National Observatory, Arizona, USA; the McDonald Observatory, Texas, USA; the South Africa Astronomical Observatory, South Africa; the Wise Observatory, Israel and the Piszkéstető Observatory, Hungary.