  The stability of the flux line lattice has been investigated within
anisotropic London theory. This is the first full-scale investigation of
instabilities in the `chain' state. It has been found that the lattice is
stable at large fields, but that instabilities occur as the field is reduced.
The field at which these instabilities first arise, $b^*(\epsilon,\theta)$,
depends on the anisotropy $\epsilon$ and the angle $\theta$ at which the
lattice is tilted away from the $c$-axis. These instabilities initially occur
at wavevector $k^*(\epsilon,\theta)$, and the component of $k^*$ along the
average direction of the flux lines, $k_z$, is always finite. As the
instability occurs at finite $k_z$ the dependence of the cutoff on $k_z$ is
important, and we have used a cutoff suggested by Sudb\ospace and Brandt. The
instabilities only occur for values of the anisotropy $\epsilon$ appropriate to
a material like BSCCO, and not for anisotropies more appropriate to YBCO. The
lower critical field $H_{c_1}(\phi)$ is calculated as a function of the angle
$\phi$ at which the applied field is tilted away from the crystal axis. The
presence of kinks in $H_{c_1}(\phi)$ is seen to be related to instabilities in
the equilibrium flux line structure.
