Should Power of attorney to company's employee for tax matter be stamped

The managing director is the signatory of Income Tax return as per IT Act. accordingly he is supposed to attend hearing before tax authorities. However due to lack of time , he wants to authorize me to represent, attend case before income tax authorities.

my question is - should authorization letter should be duly executed in required stamp paper / or stamped?

or simple authorisation letter mentioning me as employee of the company is legally valid and acceptable