  We investigate the stability to nonaxisymmetric perturbations of an accretion
disc in which a poloidal magnetic field provides part of the radial support
against gravity. Interchange instability due to radial gradients in the
magnetic field are strongly stabilized by the shear flow in the disc. For
smooth field distributions this instability is restricted to discs in which the
magnetic energy is comparable to the gravitational energy. An incompressible
model for the instability akin to the Boussinesq approximation for convection
is given which predicts the behaviour of the instability accurately. Global
axisymmetric disturbances are also considered and found to be stable for a
certain class of models. The results indicate that accretion discs may be able
to support poloidal fields which are strong enough to suppress other forms of
magnetic instability. These strong and stable field distributions are likely to
be well suited for the magnetic acceleration of jets and winds.
