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 \sim 64 K ( \sigma _ { T } = 104 K , N = 332 dwarfs ) and \sim 54 K with a \sigma _ { T } = 131 K ( N = 202 giants ) .
The same comparison with the sample of RM05 for stars with [ Fe/H ] < -2.5 provides a difference of \sim - 87 K ( \sigma _ { T } = 194 K , N = 12 dwarf stars ) and \sim 61 K ( \sigma _ { T } = 62 K , N = 18 giant stars ) .

Conclusions : Our temperature scale is slightly hotter than that of AAM96 and RM05 for metal-rich dwarf stars but cooler than that of RM05 for metal-poor dwarfs .
We have performed an 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 over others available in the literature .
In our implementation we avoid the transformation of the 2MASS JHK _ { s } magnitudes to a different photometric system and thus fully exploit the excellent internal consistency of the 2MASS photometric system .