  Measurement of the integrated luminosity at the International Linear Collider
(ILC) will be accomplished by counting the rate of small angle Bhabha
scattering events. The physics requirements for ILC set the constraint on the
relative precision of the luminosity measurement to be of a permille order. The
required precision can be achieved by construction of a fine granulated
electromagnetic calorimeter of high energy and polar angle resolution and by
sufficient experimental control of numerous systematic effects. One of the
leading systematic effects in luminosity measurement is the background
originating from four-fermion processes, referred to as the physics background.
In this paper a possible selection strategy to measure the luminosity is
proposed from the perspective of optimal signal to background separation.
