George Walden, former education minister and Tory MP for Buckingham, describes Baroness Thatcher in his autobiography as a "sad sight" who has not taken well her loss of power.
Mr Walden, who has been one of the Conservatives' sternest critics since he stood down at the last election, said he noticed her behaviour on a trip to the University of Buckingham.
"[She] was reaching the age where her over-immaculate hair set off the ravages of her face ... She was also drinking too much," he writes in his book, Lucky George.
"In the old days she would put down a whisky soda or two late in the evening, after her umpteen-hour day; now she seemed to have had a little too much before dinner."
Mr Walden goes on to describe another incident in which he noticed how much alcohol she had been drinking and her increasingly eccentric manner.
"One evening at a buffet dinner with an Eastern European prime minister, she was on a drink-fuelled high," he writes in the book, which is published next week.
"The most vigorous and intelligent prime minister we had had in decades had reached a premature anecdotage, though in her case she conversed in quotations from herself, from `Winston', or from some founding father from America."
Coming just days before the 20th anniversary of her election to power, the revelations are guaranteed to infuriate the Thatcher family and her most loyal followers in the Conservative Party.
Mr Walden, whose book is subtitled Memoirs of An Anti-Politician, is likely to come under intense criticism from his former colleagues in the Commons.Reuse content