  We studied the proton-rich $T_z=-1$ nucleus $^{70}$Kr through inelastic
scattering at intermediate energies in order to extract the reduced transition
probability, $B(E2;\;0^+ \rightarrow 2^+)$. Comparison with the other members
of the $A=70$ isospin triplet, $^{70}$Br and $^{70}$Se, studied in the same
experiment, shows a $3\sigma$ deviation from the expected linearity of the
electromagnetic matrix elements as a function of $T_z$.
  At present, no established nuclear structure theory can describe this
observed deviation quantitatively. This is the first violation of isospin
symmetry at this level observed in the transition matrix elements. A heuristic
approach may explain the anomaly by a shape change between the mirror nuclei
$^{70}$Kr and $^{70}$Se contrary to the model predictions.
