  The work shows that a single elongated island immersed in the quasi-neutral
current sheet makes it MHD unstable. Typical values of growth rate are found to
be several per cent of inverse Alfven time for broad sheets. Hall dynamics
greatly enhance instability and growth rate reaches one-tenth of ion cyclotron
frequency when sheet width is comparable to ion inertia length. A weak
square-root dependence of increment on island width and length is derived both
from numerical simulation and analytical analysis. At the non-linear phase of
evolution a phenomenon of fast impulsive intensification of current is found.
After finite time of about 10 ion-cyclotron periods it ends by a collapse of
the sheet. It is shown that Hall dynamics is responsible for such a behaviour.
Possible implications for magnetotail substorms are discussed.
