  Within the complex phase diagram of the hole-doped cuprates, seizing the
nature of the mysterious pseudo-gap phase is essential to unravel the
microscopic origin of high-temperature superconductivity. Below the pseudo-gap
temperature $\rm T^{\star}$, evidences for intra-unit-cell orders breaking the
4-fold rotation symmetry have been provided by neutron diffraction and scanning
tunneling spectroscopy. Using polarized neutron diffraction on a detwinned $\rm
YBa_2Cu_3O_{6.6}$ sample, we here report a distinct a-b anisotropy of the
intra-unit-cell magnetic structure factor below $\rm T^{\star}$, highlighting
that intra-unit-cell order in this material breaks the mirror symmetry of the
CuO$_2$ bilayers. This is likely to originate from a crisscrossed arrangement
of loop currents within the $\rm CuO_2$ bilayer, resulting in a bilayer mean
toroidal axis along the $\rm {\bf b}$ direction.
