  We report the discovery of an X-ray transient, observed in outburst with
XMM-Newton on March 20, 2003, and with position 18h12m27.8s, -18 12 34 (J2000,
approximate positional error 2"). No known source is present at this position
and the source was not detected during published ROSAT or ASCA observations of
that region. However, the source may be associated with 1H1812-182 detected by
HEAO 1, although the error bars on the HEAO 1 position are very large and the
two sources could also be unrelated. Therefore, we name the source XMMU
J181227.8-181234. Initially, the source was not detected using the All-Sky
Monitor (ASM) on-board the Rossi X-ray Timing Explorer, however, reprocessing
of the ASM data shows that the source was in fact detected and it was active
for about 50 days. The X-ray spectrum of this transient is fitted equally well
by an absorbed power-law (with a spectral index of 2.5) or multi-colour disk
blackbody model (with kT ~ 2 keV), where we find that the source is highly
absorbed. We detect an unabsorbed 0.5-10 keV flux in the range (2-5)E-9 ergs
cm^{-2} s^{-1}, which at a distance of 8 kpc corresponds to a 0.5-10 keV
luminosity of (1-4)E37 ergs s^{-1}. No pulsations were detected by timing
analysis. A colour-colour diagram from ASM data of different accreting objects
suggests that the transient is a high-mass X-ray binary, as is also suggested
by the high absorption compared to the average interstellar value in the
direction of the source. However, the power-law spectral index is far more
typical of a low-mass X-ray binary. Thus, we are unable to conclusively
identify the nature of the transient. We also report on three sources first
detected by the ASCA Galactic Plane Survey that are close to this transient.
