In addition to the portrait, a collection of letters written by Gandhi to freedom fighter Sarat Chandra Bose, who is the elder brother of Subhash Chandra Bose, fetched 37,500 pounds at the Sotheby's auction.
The sketch shows Gandhi writing with a look of intense concentration whilst seated on the floor, and is inscribed by him with the words "Truth is God/MK Gandhi/4.12.'31'."
"Gandhi would not sit for formal photographs, let alone a portraitist, so Amshewitz had to sketch him at his daily tasks," said the auction house in a statement.
The pencil drawing is one of a number of portraits created by artist John Henry Amshewitz when Gandhi was visiting London for the Round Table Conference in 1931.
Founder of Serum Institute of India, Mr Poonawalla added that he wanted to bring the historic and rare work back to India.
"Mahatma Gandhi is one of the most respected and revered figures in the world and considered to be the father of our nation, and so to have such a rare and intimate depiction of him at work, and signed by the great man himself, is an incredible honour and an absolute pleasure," Mr Poonawalla added.
(With inputs from PTI and IANS)