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The Grójec Fault (GF) is of significant interest due to its orientation perpendicular to the dominant NW-SE structural grain in the transition zone from Precambrian to Phanerozoic Europe. The fault is located along the extension of the boundary between the Sarmatia and Fennoscandia segments of the East European Craton (EEC). While Late Cretaceous fault activity has been documented by studies since the 1980s, its earlier history remains poorly understood. To better understand the deep crustal structure and its link to shallow deformation, new magnetotelluric data from 39 points, along with older geomagnetic data from 22 survey points, were used to create a three-dimensional (3-D) model of the geoelectric structure. The ModEM 3-D inversion code was used to interpret the data and create a model of resistivity distribution. The magnetotelluric and auxiliary gravimetric data show a continuation of the suture between Fennoscandia and Sarmatia in the substratum of the Palaeozoic Platform, indicating the SW-ward extension of basement belonging to the EEC. The data reveal activity of the GF and coinciding suture zone during early Permian rifting and post-rift Mesozoic subsidence, manifested by transverse segmentation of the Mid-Polish Trough (MPT). This evidence, in combination with previously documented strike-slip activity of the GF during Late Cretaceous basin inversion, points to a long-lasting crustal weakness zone associated with the GF. A significant result of the modelling was the identification of a large, highly-conducting anomaly in the Earth's crust visible from a depth of 8 km and extending in the NW-SE direction beneath the axis of the MPT. The anomaly suggests fluid migration from the upper mantle causing the precipitation of conducting minerals during the rifting phase, mainly sulphides and graphite. • The Grójec Fault is the SW-ward extension of the Fennoscandia-Sarmatia suture. • The fault was activated during Permian rifting and Late Cretaceous basin inversion. • Fault activity was manifested by transverse segmentation of the Mid-Polish Trough. • The Grójec Fault represents a long-lasting crustal weakness zone. • A highly-conducting anomaly extends beneath the axis of the Mid-Polish Trough. [ABSTRACT FROM AUTHOR]