Lord Ashcroft, the co-author of a controversial biography of David Cameron, missed the book's launch party after suffering from a life-threatening illness.
Call Me Dave, written with journalist Isabel Oakeshott, contained a series of embarrassing claims about the Prime Minister's student days.
One, which was dismissed as nonsense by sources close to the premier, was the allegation that he participated in a stunt while at Oxford University involving a dead pig and "a private part of his anatomy".
Guests at the launch event in London were told that Lord Ashcroft had been diagnosed with "life threatening" septic shock, liver and kidney failure and spent 18 days in intensive care.
A special report by Channel 5 News in Belize, where Lord Ashcroft grew up and has close ties, reported he first fell ill with a severe stomach pain in Croatia on September 22, and his condition deteriorated after flying to the Turks and Caicos the following day.
On September 24 he was transferred to the Cleveland Clinic in Ohio, USA where he was diagnosed with septic shock, according to the news report which was shown at the book launch event.
A spokeswoman said Lord Ashcroft was now recovering and was "out of danger".
In his speech to the event, delivered on his behalf by publisher Iain Dale, Lord Ashcroft said: "I have been a little pre-occupied for the past four weeks. I haven't, of course, set foot in the UK since my illness but I am reliably informed that my book has caused a little bit of a stir."
He added that there had been "h ysterical, ill-informed and illogical criticisms" of his book but insisted it was "wide of the mark" to claim the biography was motivated by "revenge and malice".
Lord Ashcroft said "while I am certainly mischievous, I am not malicious".
In a reference to the pig's head story, Lord Ashcroft joked that he had "an apology" to make.
" I read in The Independent, at the height of the furore, that management up and down the country were so worried by their workforce's fascination with 'pig-gate', particularly on social media, that they fear productivity will be affected.
"All I can say if that if the UK's GDP for the second half of this year really does dip significantly, then I am very sorry and I take full responsibility for it."
But he added that "for various reasons" he and Ms Oakeshott decided to l eave out "a great deal of material out of the book relating to David Cameron's private life".