We calculate a marginal order parameter dimension $m_c$ which in a weakly diluted quenched $m$-vector model controls the crossover from a universality class of a ``pure model ($m>m_c$) to a new universality class ($m<m_c$). Exploiting the Harris criterion and the field-theoretical renormalization group approach allows us to obtain $m_c$ as a five-loop $epsilon$-expansion as well as a six-loop pseudo-$epsilon$ expansion. In order to estimate the numerical value of $m_c$ we process the series by precisely adjusted Pade--Borel--Leroy resummation procedures. Our final result $m_c=1.912pm0.004<2$ stems from the longer and more reliable pseudo-$epsilon$ expansion, suggesting that a weak quenched disorder does not change the values of $xy$-model critical exponents as it follows from the experiments on critical properties of ${rm He}^4$ in porous media.
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