Context: The effective temperature scale of FGK stars, especially at the lowest metallicities remains a major problem in the chemical abundance analysis of metal-poor stars.
Aims: We present a new implementation of the infrared flux method (IRFM) using the 2MASS catalogue.
Methods: We computed the theoretical quantities in the 2MASS JHK_s filters by integrating theoretical fluxes computed from ATLAS models, and compare them directly with the observed 2MASS JHK_s magnitudes. This is the main difference between our implementation of the IRFM and that of Ramírez & Meléndez (2005, ApJ, 626, 446, hereafter RM05), since to introduce new stars at the lowest metallicities they transform the 2MASS JHK_s magnitudes into the TCS photometric system. We merge in our sample the stars from Alonso et al. (1996, A&AS, 117, 227, hereafter AAM96; 1999, A&AS, 139, 335, hereafter AAM99), and other studies to appropriately cover a wide range of metallicities, ending up with 555 dwarf and subgiant field stars and 264 giant field stars. We derived a new bolometric flux calibration using the available Johnson-Cousins UBV(RI)_C and the 2MASS JHK_s photometry. We also computed new T_eff versus colour empirical calibrations using our extended sample of stars.
Results: We derived effectives temperatures for almost all the stars in the AAM96 and AAM99 samples and find that our scales of temperature are hotter by ˜ }64 K (sigma_T=104 K, N=332 dwarfs) and {˜54 K with a sigma_T=131 K (N=202 giants). The same comparison with the sample of RM05 for stars with [Fe/H]