  We study, by means of a variational method, the stability of a condensate in
a magnetically trapped atomic Bose gas with a negative scattering length and
find that the condensate is unstable in general. However, for temperatures
sufficiently close to the critical temperature the condensate turns out to be
metastable. For that case we determine in the usual WKB approximation the decay
rate of the condensate due to macroscopic quantum fluctuations. When
appropriate, we also calculate the decay rate due to thermal fluctuations. An
important feature of our approach is that (nonsingular) phase fluctuations of
the condensate are taken into account exactly.
