A large are a window less gas flow multi wire proportional counting system for the calibration of large area reference sources has been developed as a primary standard at Bhabha Atomic Research Centre (BARC). The counting system consists of a multi wire proportional counter (MWPC), vacuum system, gas flow system and pulse process sing units. The MWPC detector assembly consists of a vacuum tight aluminum enclosure, multi wire grid and sliding sourcetray. Various detector characteristics like operating characteristics curve, Fe-55 spectrum for beta discriminator threshold setting and dead time of the measurement  system were studied and determined in order to achieve an optimized detection cap- ability. The surface emission rates of different source strengths were measured and their relative combined standard uncertainties were determined. Large Area Sources Comparison Exercise(LASCE) was organized by International Committee on Radio nuclide Metrology (ICRM) working group and co- ordinate by National Institute for Ionising Radiation Metrology (ENEA), Italy, to demonstrate equivalence of surface emission rate measurements at the international platform. BARC participated in the programme and the results of LASCE are also discussed in this paper.