  The Cosmic Microwave Background radiation is a fundamental prediction of Hot
Big Bang cosmology. The temperature of its black-body spectrum has been
measured at the present time, $T_{\rm CMBR,0}$ = 2.726$\pm$ 0.010 K, and is
predicted to have been higher in the past. At earlier time, the temperature can
be measured, in principle, using the excitation of atomic fine structure levels
by the radiation field. All previous measurements however give only upper
limits as they assume that no other significant source of excitation is
present. Here we report the detection of absorption from the first {\sl and}
second fine-structure levels of neutral carbon atoms in an isolated remote
cloud at a redshift of 2.33771. In addition, the unusual detection of molecular
hydrogen in several rotational levels and the presence of ionized carbon in its
excited fine structure level make the absorption system unique to constrain,
directly from observation, the different excitation processes at play. It is
shown for the first time that the cosmic radiation was warmer in the past. We
find 6.0 < T_{\rm CMBR} < 14 K at z = 2.33771 when 9.1 K is expected in the Hot
Big Bang cosmology.
