A system for radially expanding a tubular element (2) in an underground borehole (3) comprises an expansion string (16) extending into the tubular element (2) and including an expander (27) and a jack device (24) for pulling the expander (27) through the tubular element (2) so as to radially expand the tubular element (2). The jack device (24) is provided with an anchor (28) having at least one slip element (50) adapted to be radially moved against the inner surface of the tubular element (2). A cage (14) is positioned above the tubular element (2), which cage (14) is surrounded by a cylindrical wall and adapted to receive the anchor (28) and to be radially expanded by the anchor (28) against said cylindrical wall.