We report the high-pressure structure-related properties of a ferroelectric heterometallic formate perovskite framework templated by the ethylammonium cation (CH3CH2NH3+, EtA+). High-pressure X-ray diffraction studies show a first-order structural phase transition at 3.6(2) GPa from the polar Pn ambient phase to a centrosymmetric P21/n high-pressure phase. A high-pressure Raman scattering experiment indicates the same transition in the 4.0-4.4 GPa range. The mechanism of the phase transition involves strong compression and distortion of the NaO6 subnetwork and a decrease in the space available for the accommodated EtA+ cations, resulting in a change in their configuration within the pores at 3.7 GPa. Using density functional theory the value of the ferroelectric polarisation within the ac plane is calculated to be 0.9 μC cm-2 at ambient pressure, increasing in magnitude to a value of 1.1 μC cm-2 at a pressure of 3 GPa before vanishing at the phase transition.