The cap is white and fairly smooth, and quickly turns yellow when cut or bruised; the base of the stem also turns yellow when cut.  Young specimens often have a squareish appearance viewed from above.  Spores are dark brown.

There are several similar species that slowly turn yellow when cut or bruised

Photograph from top down, side and underneath; your photos should show yellow staining on the cap and stem base.

Grassland, parks etc and under broadleaved trees.

Late summer to late autumn.

Fairly common and widespread in Britain.

Fairly frequent in Leicestershire and Rutland.

https://www.first-nature.com/fungi/agaricus-xanthodermus.php