The 65-year-old reportedly collapsed into his chair just five minutes into a speech in central London today.
He told an audience at The Oldie Literary Lunch: “I’m sorry I can’t go on.”
He is believed to be receiving treatment in the nearby University College hospital.
Mr Morton has recently released a book about Meghan Markle but was speaking to promote his book on American socialite Wallis Simpson, who later married Prince Edward to become the Duchess of Windsor.
A source told the Sun: “Andrew was five minutes into his speech about Wallis Simpson when he lost concentration.
“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 and taken to UCLH.
“People are saying it’s a suspected stroke.”
Mr Morton was scheduled to appear at another event at a bookshop in West London this evening, but this has now been cancelled.
Waterstones King’s Road tweeted: “Due to unforeseen circumstances, we have been forced to cancel the Andrew Morton event this evening. Please accept our sincerest apologies. We are offering refunds to all ticketholders so do pop in. Many thanks.”
The author was also due to help with American network ABC’s coverage of Saturday’s royal wedding.
Former Fleet Street journalist Mr Morton has penned a series of unofficial biographies of high-profile celebrities, including Tom Cruise and Angelina Jolie.
His biographies on the royal family include works on Princess Diana and Meghan Markle.