The royal, who was once reportedly on 20 Marlboro Lights per day, remained ciggy-free for a year but was alleged to have given in to the puff over stress from his training with the Royal Air Force.
"He has been feeling the pressure, and as a result he has turned back to cigs," News of the World quoted an insider as saying.
"Harry knows that it doesn't look good to younger people who look up to him. He is really trying to quit," the insider added.
The 24-year-old was also said to be following the book and CD that comes with the program.