The present invention provides a solid organomagnesium precursor having formula {Mg(OR′)X}.a{MgX2}.b{Mg(OR′)2}.c{R′OH}, wherein R′ is selected from a hydrocarbon group, X is selected from a halide group, and a:b:c is in range of 0.01-0.5:0.01-0.5:0.01-5 and process for preparing the same, said process comprising contacting a magnesium source with a solvating agent, an organohalide and an alcohol to obtain the solid organomagnesium precursor. The present invention also provides a process for preparing a catalyst system using the organomagnesium precursor and its use thereof for polymerization of olefins. The organomagnesium precursor is prepared as follows: At 0° C., magnesium in diethyl ether is reacted with the organohalide. After all magnesium has reacted, the calculated amount of alcohol was added and after the completion of addition, the ether was evaporated and a solid compound obtained.