Mr Self was sacked from the Observer after the allegation surfaced last week, but issued a strenuous denial through his lawyers.
However, in an exclusive interview with this paper on Friday night, he admitted to snorting heroin in the toilet on John Major's jet while part of the Prime Minister's election campaign press party on a flight from London to the East Midlands a week last Thursday.
The self-confessed former heroin addict went to the toilet during the flight and snorted a line of heroin along the side of the wash basin. He said: "So I was smacked out on the Prime Minister's jet - big deal."
Mr Self's astonishing disclosure is bound to delight the Conservatives who are desperate for any ammunition with which to hit back at sleaze allegations.
The self-styled literary enfant terrible said he had denied the charge because he thought there would be no evidence to support it.
He revealed that he had also initially denied it to Will Hutton, editor of the Observer, which had led Mr Hutton to persuade another newspaper to withdraw a story about him last Sunday.
But when he was put under further pressure to sign an affidavit, he admitted to Mr Hutton and the Observer's editor-in-chief, Alan Rusbridger, that the charge was true, and he was then sacked.
He could not bring himself to put the denial in a formal, legal document for fear that, if found out, the embarrassment to the Observer and its sister paper, the Guardian, would be even worse, he said. One of their journalists not only would have taken heroin on Mr Major'sflight but also would have committed perjury.
"I confessed up to Will Hutton ... that I'd had a bad patch with smack and as it happened I did take a little," said Mr Self. "There was pandemonium. Rusbridger was called in and they decided to fire me."
Mr Self thinks another journalist sitting at the back of the aircraft either spotted the change in his manner after taking the drug, or that he may have left a small amount in the toilet.
Mr Self, who left Londonyesterday to go to the country, said he was fed up with the hypocrisy, both at the liberally-minded Observer, which promoted him as a writer who took drugs, and among the press generally.
Mr Hutton said yesterday that Mr Self had confessed to him and that he had offered the writer rehabilitation treatment which had been refused.