We study the bulk and finite-size critical behavior of the O(n) symmetric ϕ 4 theory with spatially anisotropic interactions of non-cubic symmetry in d < 4 dimensions. In such systems of a given (d, n) universality class, two-scale factor universality is absent in bulk correlation functions, and finite-size scaling functions including the Privman-Fisher scaling form of the free energy, the Binder cumulant ratio and the Casimir amplitude are shown to be nonuniversal. In particular it is shown that, for anisotropic confined systems, isotropy cannot be restored by an anisotropic scale transformation.PACS numbers: 05.70. Jk, A basic tenet in the physics of critical phenomena is the notion of a universality class. It is characterized by the dimensionality d of the system and by the number n of the components of the order parameter. (See, e.g., the review article [1].) Within a certain (d, n) universality class, the universal quantities (critical exponents, amplitude ratios and scaling functions) are independent of microscopic details, such as the particular type of (finiterange or van der Waals type) interactions or the lattice structure [2]. This implies that a given universality class includes both spatially isotropic and anisotropic systems.Once the universal quantities of a universality class are known the asymptotic critical behavior of very different systems (e.g., fluids and magnets) is believed to be known completely provided that only two nonuniversal amplitudes A 1 and A 2 are given. This property is known as two-scale factor universality or hyperuniversality [1,3,4]. In terms of the singular part of the reduced bulk free energy densitywith W (0) = 1 and t = (T − T c )/T c ≪ 1, this property can be stated asThus the amplitude ξ 0 = (Q/A 1 ) 1/d of the correlation length ξ = ξ 0 t −ν at zero ordering field h is not an independent amplitude but is universally related to A 1 . The validity of two-scale factor universality has been established by the renormalization-group (RG) theory on the basis of an isotropic Hamiltonian with short-range interactions below the upper critical dimension d * = 4 [4] but no general proof has been given for the anisotropic case.In this paper we study the critical behavior of systems with a spatial anisotropy of non-cubic symmetry within a given (d, n) universality class. An example is an Ising ferromagnet with an isotropic nearest-neighbor (NN) coupling J > 0 and an anisotropic next-nearestneighbor (ANNN) coupling J ′ on a simple-cubic lattice. In some range of J ′ /J this model has the same type of critical behavior as the ordinary (J ′ = 0) Ising model. We shall show that for such systems Eq. (2) must be generalized towhere 0 , A 2 whose ratios are also nonuniversal. Note that there still exists a unique critical exponent ν(d, n) that is identical for isotropic and anisotropic systems within the same (d, n) universality class [6,7,8,9,10].A different type of critical behavior exists in the socalled strongly anisotropic systems [11,12,13,14] where not only amplitudes depend on t...