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The COMPASS experiment at CERN has collected a large sample of events ofinelastic scattering of longitudinally polarised muons off longitudinallypolarised protons in the non-perturbative region (four-momentum transfersquared $Q^2<1$ GeV$^2$/$c^2$), with a Bjorken scaling variable in the range$4\times 10^{-5}<x<4\times 10^{-2}$. The data set is two orders of magnitudelarger than the similar sample collected by the SMC experiment. These datacomplement our data for polarised deuterons. They allow the accuratedetermination of the longitudinal double spin asymmetry $A_1^p$ and of thespin-dependent structure function $g_1^p$ of the proton in the region of low$x$ and low $Q^2$. The preliminary results of the analysis of these data yieldnon zero and positive asymmetries and of the structure function $g_1^p$. Thisis the first time that spin effects are observed at such low $x$.

Longitudinal double spin asymmetry $A_1^p$ and spin-dependent structure function $g_1^p$ of the proton at low $x$ and low $Q^2$ from COMPASS