  The critical behaviour of systems belonging to the three-dimensional Ising
universality class is studied theoretically using the collective variables (CV)
method. The partition function of a one-component spin system is calculated by
the integration over the layers of the CV phase space in the approximation of
the non-Gaussian sextic distribution of order-parameter fluctuations (the
\rho^6 model). A specific feature of the proposed calculation consists in
making allowance for the dependence of the Fourier transform of the interaction
potential on the wave vector. The inclusion of the correction for the potential
averaging leads to a nonzero critical exponent of the correlation function \eta
and the renormalization of the values of other critical exponents. The
contributions from this correction to the recurrence relations for the \rho^6
model, fixed-point coordinates and elements of the renormalization-group linear
transformation matrix are singled out. The expression for a small critical
exponent \eta is obtained in a higher non-Gaussian approximation.
