We disclose the vertically aligned deformed helix ferroelectric liquid crystal (VADHFLC) whose Kerr constant ($K_{\mathrm{kerr}}≈ 130$~nm/V$^2$ at $λ=543$~nm) is one order of magnitude higher than any other value previously reported for liquid crystalline structures. Under certain conditions, the phase modulation with ellipticity less than 0.05 over the range of continuous and hysteresis free electric adjustment of the phase shift from zero to 2$π$ have been obtained at sub-kilohertz frequency. ; Comment: revtex4-1, 4 pages, 5 figures