Dear Meghna,
Punishment for a defaulter is nothing but one of the means to improve the behavior of an employee. "Demotion" is one such punishment.
However, before awarding such punishment, make sure that the culpability of the employee is established. What efforts has management taken to improve the performance of the employee? What efforts has his reporting manager taken to improve the performance of the employee?
You could have given a little more detail about the case, such as the designation of the employee, how long they have been working, what the under-performance issues were, etc. What counseling was done to improve their performance, and how many warning letters were given to improve their performance?
Any demotion results in a reduction in salary. If you demote the employee, make sure not to reduce their basic salary. Keep the basic salary as it is. Secondly, demote the employee by issuing a proper letter to them. If you want to see a draft of such a letter, you can click here. You need to make a few amendments to this draft.
On the flip side, the motivation level of the demoted employee may decrease. They may become the target of ridicule and possibly decide to quit the company on their own.
Regards,
DVD