Controlling spin current and magnetic exchange coupling by applying an electric field and achieving high spin injection efficiency at the same time in a nanostructure coupled to ferromagnetic electrodes have been the outstanding challenges in nanoscale spintronics. A relentless quest is going on to find new low-dimensional materials with tunable spin dependent properties to address these challenges. Herein, we predict, from first-principles, the transverse-electric-field induced switching in the sign of exchange coupling and tunnel magneto-resistance in a boron nitride nanotube quantum dot attached to ferromagnetic nickel contacts. An orbital dependent density functional theory in conjunction with a single particle Green's function approach is used to study the spin dependent current. The origin of switching is attributed to the electric field induced modification of magnetic exchange interaction at the interface caused by the Stark effect. In addition, spin injection efficiency is found to vary from 61% to 89% depending upon the magnetic configurations at the electrodes. These novel findings are expected to open up a new pathway for the application of boron nitride nanotube quantum dots in next generation nanoscale spintronics.