This paper considers force-free equilibria in twisted magnetic flux tubes with helical symmetry. Such symmetry simplifies the force-free field condition to a pair of equations which are generalisations of the Grad-Shafranov equations. A simple linear solution is given analytically in terms of a Heun function, which exhibits several important aspects of twisted flux tubes. The equilibrium equations impose a functional relation between the axial field component and the flux function for the other two components, which can be used to establish current-sheet formation when certain classes of flux tubes are twisted together. Implications for magnetic energy release in the solar corona are discussed.