Context :

Aims : The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts .
Whether the extra light arises from the X-ray heated outer disc , from the inner hot accretion flow , or from the jet is currently debated .
Optical polarisation measurements are able to distinguish the relative contributions of these components .

Methods : We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018 .

Results : We detect small , \sim 0.7 % , but statistically significant polarisation , part of which is of interstellar origin .
Depending on the interstellar polarisation estimate , the intrinsic polarisation degree of the source is between \sim 0.3 % and 0.7 % , and the polarisation position angle is between \sim 10 \degr - 30 \degr .
We show that the polarisation increases after MJD 58222 ( 2018 April 14 ) .
The change is of the order of 0.1 % and is most pronounced in the R band .
The change of the source Stokes parameters occurs simultaneously with the drop of the observed V -band flux and a slow softening of the X-ray spectrum .
The Stokes vectors of intrinsic polarisation before and after the drop are parallel , at least in the V and R filters .

Conclusions : We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component , which we associate with the the hot flow or jet emission .
The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense , ionised , and magnetised medium close to the black hole .
The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow .