reduction of basic without prior intimation

Can a pvt ltd company reduce employee basic pay to half after six months of working without informing the employees? This has basically been done to reduce employer's contribution to provident fund. But due to slashing of basic pay the salary structure has been modified to that mentioned in the contract entered while joining the company. No fresh structure has been issued against the new salary structure niether the employees recieved any prior intimation. I wish to know this that whether company can legally do such a thing? and if my salary structure has changed without any prior notice does that make my contract null and void?