In the present study, we revise the limit on the distance at which ionisation of the hydrogen atom occurs under the influence of an antiproton. This distance, known as the Fermi-Teller radius (RFT=0.639315 Bohr radii), has been the subject of detailed study until recently. The various theoretical methods showed excellent agreement with each other. However, the main conclusion following from the description of the motion of the electron in the dipole field in the presence of an external magnetic field is that the limiting value below which there is no binding of the electron cannot be found. Within the numerical method of solving the Dirac equation with a two-centre interaction potential, we find that RFT is different from zero for fields from 10−4 to 104 Tesla. On this basis, estimates of the principal quantum number for the resulting state of the protonium atom (Pn) are given.