  $^{\rm {180m}}$Ta is the rarest naturally occurring quasi-stable isotope and
the longest lived metastable state which is known. Its possible decay via the
$\beta^-$ or the electron capture channel has never been observed. This article
presents a search for the decay of $^{\rm {180m}}$Ta with an ultra low
background Sandwich HPGe gamma spectrometry setup in the HADES underground
laboratory. No signal is observed and improved lower partial half-life limits
are set with a Bayesian analysis to $5.8\cdot10^{16}$ yr for the $\beta^-$
channel and $2.0\cdot10^{17}$ yr for the electron capture channel (90%
credibility). The total half-life of $^{\rm {180m}}$Ta is longer than
$4.5\cdot10^{16}$ yr. This is more than a factor of two improvement compared to
previous searches.
