  We present a new implementation of the infrared flux method (IRFM) using the
2MASS catalogue. We compute the theoretical quantities in the 2MASS JHKs
filters by integrating theoretical fluxes computed from ATLAS models, and
compare them directly with the observed 2MASS JHKs magnitudes. This is the main
difference between our implementation of the IRFM and that of Ramirez &
Melendez, since to introduce new stars at the lowest metallicities they
transform the 2MASS JHKs magnitudes into the TCS photometric system. We merge
in our sample the dwarfs stars from Alonso and collaborators with the stars in
the sample of Ramirez & Melendez, Casagrande et al., Christlieb et al. and
Bonifacio et al. The final number of stars in our sample is 555 dwarf and
subgiant field stars, and 264 giant field stars. We derive a new bolometric
flux calibration using the available Johnson-Cousins UBVRI and the 2MASS JHKs
photometry. We also compute new Teff versus colour empirical calibrations using
our extended sample of stars. We derive effectives temperatures for almost all
the stars in the Alonso et al. sample and find that our scales of temperature
are hotter by ~64K(dT=104K,N=332 dwarfs) and ~54K(dT=131K,N=202 giants). The
same comparison with the sample of Ramirez & Melendez for stars with
[Fe/H]<-2.5 provides a difference of ~-87K(dT=194K,N=12 dwarfs) and
~61K(dT=62K,N=18 giants). Our temperature scale is slightly hotter than that of
Alonso et al. and Ramirez & Melendez for metal-rich dwarf stars but cooler than
that of Ramirez & Melendez for metal-poor dwarfs. We have performed a fully
self-consistent IRFM in the 2MASS photometric system. For those who wish to use
2MASS photometry and colour-temperature calibrations to derive effective
temperatures, especially for metal-poor stars, we recommend our calibrations.
