  The coexistence of weak ferromagnetism and superconductivity in ErNi$_{2}$B$%
_{2}$C suggests the possibility of a spontaneous vortex phase (SVP) in which
vortices appear in the absence of an external field. We report evidence for the
long-sought SVP from the in-plane magnetic penetration depth $\Delta \lambda
(T)$ of high-quality single crystals of ErNi$_{2}$B$_{2}$C. In addition to
expected features at the N\'{e}el temperature $T_{N}$ = 6.0 K and weak
ferromagnetic onset at $T_{WFM}=2.3 $K, $\Delta \lambda (T)$ rises to a maximum
at $T_{m}=0.45$ K before dropping sharply down to $\sim $0.1 K. We assign the
0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A
model proposed by Koshelev and Vinokur explains the increasing $\Delta \lambda
(T)$ as a consequence of increasing vortex density, and its subsequent decrease
below $T_{m}$ as defect pinning suppresses vortex hopping.
