Using stacks of Lyα images of 2128 Lyα emitters (LAEs) and 24 proto-cluster UV-selected galaxies (LBGs) at z = 3.1, we examine the surface brightness profiles of Lyα haloes around high-z galaxies as a function of environment and UV luminosity. We find that the slopes of the Lyα radial profiles become flatter as the Mpc-scale LAE surface density increases, but that they are almost independent of the central UV luminosity. The characteristic exponential scalelength of the Lyα haloes appears to be proportional to the square of the LAE surface density (). Including the diffuse, extended Lyα haloes, the rest-frame Lyα equivalent width of the LAEs in the densest regions approaches EW0 ∼ 200 Å, the maximum value expected for young (<107 yr) galaxies. This suggests that Lyα photons formed via shock compression by gas outflows or cooling radiation by gravitational gas inflows may partly contribute to the illumination of Lyα haloes; however, most of their Lyα luminosity can be explained by photoionization by or by scattering of Lyα photons produced from H ii regions in and around the central galaxies. Regardless of the source of Lyα photons, if the Lyα haloes trace the overall gaseous structure, following the dark matter distribution, it is not surprising that the Lyα spatial extent depends more strongly on the surrounding Mpc-scale environment than on the activity of the central galaxies.