  We study the stability of attractive atomic Bose-Einstein condensate and the
macroscopic quantum many-body tunneling (MQT) in the anharmonic trap. We
utilize correlated two-body basis function which keeps all possible two-body
correlations. The anharmonic parameter ($\lambda$) is slowly tuned from
harmonic to anharmonic. For each choice of $\lambda$ the many-body equation is
solved adiabatically. The use of the van der Waals interaction gives realistic
picture which substantially differs from the mean-field results. For weak
anharmonicity, we observe that the attractive condensate gains stability with
larger number of bosons compared to that in the pure harmonic trap. The
transition from resonances to bound states with weak anharmonicity also differs
significantly from the earlier study of Moiseyev {\it et.al.}[J. Phys. B: At.
Mol. Opt. Phys. {\bf{37}}, L193 (2004)]. We also study the tunneling of the
metastable condensate very close to the critical number $N_{cr}$ of collapse
and observe that near collapse the MQT is the dominant decay mechanism compared
to the two-body and three-body loss rate. We also observe the power law
behavior in MQT near the critical point. The results for pure harmonic trap are
in agreement with mean-field results. However we fail to retrieve the power law
behavior in anharmonic trap although MQT is still the dominant decay mechanism.
