Sinha was appointed OSD in the Prime Minister's Office on August 30 after Prime Minister Modi accepted the request of his Principal Secretary Nripendra Misra to be relieved of the post.
"The Appointments Committee of the Cabinet (ACC) has approved the appointment of PK Sinha, currently functioning as OSD in the Prime Minister's Office, as principal advisor to prime minister with effect from September 11, 2019," said a release from the ministry of personnel, public grievances and pensions.
His appointment will be co-terminus with the term of the prime minister or until further orders, whichever is earlier, the release said.
The ACC also approved the appointment of PK Mishra, who was additional principal secretary to the prime minister, as principal secretary to the prime minister with effect from September 11.