The 65-year-old was just minutes into a talk when he told an audience at Simpson's In The Strand: 'Sorry, I can't go on, ' and slumped into a chair.
He is now believed to be at the University College London Hospitals seeking urgent treatment.
"He forgot what he was saying, was sweating and started to sway".
"He ended up getting carted off from The Strand in the back of an ambulance".
"He became unwell this afternoon, he's been taken to hospital as a precautionary measure", said the spokeswoman.
However, Waterstones King's Road have since tweeted: "Due to unforeseen circumstances, we have been forced the Andrew Morton event".
Mr Morton made his name writing Diana: Her True Story, which was revised after her 1997 death in a auto crash.
The author, who has also written books about Princess Diana and Prince William, has since taken to Twitter to thank fans for their well wishes, assuring them that he is on the mend.
In 2012, while on honeymoon in South Africa, Mr Morton was seriously injured in a auto accident in which a woman and baby died.