The author, who also has a seat in the House Of Lords as a Labour peer, is known for her series of novels featuring Chief Inspector Wexford among the dozens she has written.
She had what has been described as a “serious stroke” last week and is being looked after in hospital.
A statement from her publisher, Hutchinson - Penguin Random House, said: “Ruth Rendell (Baroness Rendell of Babergh), the renowned crime writer and author of over 60 best-selling novels, suffered a serious stroke on Wednesday, 7 January.
“She is in hospital under expert care in a critical but stable condition. Her son, Simon Rendell, is with her and thanks everyone for their concern. The family request privacy while the doctors assess the best course of treatment.
“Our thoughts are with Ruth and her family at this difficult time.”
Rendell had recently completed a new novel for the company and plans remain in place to publish it in the autumn.
The 84-year-old’s most recent publication was The Girl Next Door, which came out last year along with a 50th anniversary edition of her debut novel, From Doon With Death.
Her Wexford books were dramatised for TV for more than a decade as The Ruth Rendell Mysteries, with George Baker in the lead role.
She has also published a number of books under the pen name Barbara Vine, as well as many psychological crime thrillers.
Rendell - who began her career as a local newspaper journalist but resigned after covering a tennis club annual dinner but failing to mention the death of the after-dinner speaker - was made a life peer in 1997, a year after being awarded a CBE.
Her awards have included the Crime Writers’ Association Cartier Diamond Dagger for sustained excellence in crime writing.