  We present the detection, performed with the Westerbork Synthesis Radio
Telescope (WSRT) and the Karl Jansky Very Large Array (VLA), of a fast HI 21-cm
outflow in the ultra-luminous infrared galaxy Mrk 231. The outflow is observed
as shallow HI absorption blueshifted ~1300 km/s with respect to the systemic
velocity and located against the inner kpc of the radio source. The outflowing
gas has an estimated column density between 5 and 15x10^18 Tspin cm^-2. We
derive the Tspin to lie in the range 400-2000 K and the densities are
n_HI~10-100 cm^-3. Our results confirm the multiphase nature of the outflow in
Mrk231. Although effects of the interaction between the radio plasma and the
surrounding medium cannot be ruled out, the energetics and the lack of a clear
kpc-scale jet suggest that the most likely origin of the HI outflow is a
wide-angle nuclear wind, as earlier proposed to explain the neutral outflow
traced by NaI and molecular gas. Our results suggest that an HI component is
present in fast outflows regardless of the acceleration mechanism (wind vs jet
driven) and that it must be connected with common properties of the
pre-interaction gas. Considering the observed similarity of their column
densities, the HI outflow likely represents the inner part of the broad wind
identified on larger scales in NaI. The mass outflow rate of the HI outflow
does not appear to be as large as the one observed in molecular gas. These
characteristics suggest that the HI may represent a short intermediate phase in
the rapid cooling of the gas. We also obtained deeper continuum images than
previously available. At the resolution of ~1arcsec we do not see a kpc-scale
jet. Instead, we detect a plateau of emission, likely due to star formation,
surrounding the bright nuclear region. We also detect a poorly collimated
bridge which may represent the channel feeding the southern lobe.
