Princess Margaret was admitted to hospital in London yesterday after doctors expressed concerns about her health, Buckingham Palace said.
The 70-year-old princess has been in poor health and doctors said last week that she might have suffered a minor stroke.
As a consequence of the apparent stroke, she was now suffering from "a severe loss of appetite".
A statement added: "On the advice of her doctors, Princess Margaret has been admitted to the King Edward VII hospital, where her condition can be more closely monitored."
Her few planned public engagements have been cancelled.
The princess was driven by car to the hospital in central London from Queen Elizabeth's Sandringham estate in eastern England.
There had been some hope at the end of last week that she might be responding to extra treatment and nursing care at Sandringham.