  We model the interaction between the wind from a newly formed rapidly
rotating magnetar and the surrounding progenitor. In the first few seconds
after core collapse the magnetar inflates a bubble of plasma and magnetic
fields behind the supernova shock, which expands asymmetrically because of the
pinching effect of the toroidal magnetic field, as in PWNe, even if the host
star is spherically symmetric. The degree of asymmetry depends on the ratio of
the magnetic energy to the total energy in the bubble. We assume that the wind
by newly formed magnetars inflating these bubbles is more magnetized than for
PWNe. We show that for a magnetic to total power supplied by the central
magnetar $\sim 0.1$ the bubble expands relatively spherically while for values
greater than 0.3, most of the pressure in the bubble is exerted close to the
rotation axis, driving a collimated outflow out through the host star. This can
account for the collimation inferred from observations of long-duration
gamma-ray bursts (GRBs). Given that the wind magnetization increases in time,
we thus suggest that the magnetar-driven bubble initially expands relatively
spherically (enhancing the energy of the associated supernova) while at late
times it becomes progressivelymore collimated (producing the GRB). Similar
processes may operate in more modestly rotating neutron stars to produce
asymmetric supernovae and lower energy transients such as X-ray flashes.
