The binary encounter model of Thomas and Garcia (1969) for including the effect of the net charge of the target ion has been used to obtain electron impact ionisation cross sections for alkali metal ions. Vriens' expression (1966) including exchange and interference, for the electron impact ionisation cross section has been used in the model and has been integrated over the quantum-mechanical velocity distribution for the bound electron. The calculated cross sections are found to show better agreement with available experimental results compared to other theoretical calculations.