Sir Denis Thatcher, husband of former prime minister Lady Thatcher, died today.
The 88-year old former oil businessman passed away in the Lister Hospital, London, a spokesman for the family said.
The spokesman said: "It is with deep sadness that we have to announce that Sir Denis Thatcher passed away peacefully this morning in the Lister Hospital after a short illness.
"His family were with him at his bedside when he died."
Although a non-political figure in public, Sir Denis was thought to harbour hard-right views. In 1984 he told the Swiss president at a dinner: "Keep Switzerland white."
He married the then Margaret Hilda Roberts in 1951 after having seen wartime service, and went on to become a director of Castrol.
Famously keen as a golf addict, he was best known to the public in the 1980s through the fictional parody alter-ego in Private Eye's "Dear Bill" spoof letters to his real-life friend Lord Bill Deedes, former editor of the Daily Telegraph.
Tony Blair led the tributes: "I'm very sad to hear of the death of Denis Thatcher and have sent my deepest sympathy to Lady Thatcher and his family.
"He was a successful businessman, a devoted family man, loyal friend and always entertaining company.
"It was these qualities that enabled him to provide such immensely strong support to Lady Thatcher throughout her long political career.
"He was always by her side when needed, a constant and trusted companion. He will be greatly missed by his family and his many friends in the UK and around the world."
Lady Thatcher has herself suffered from ill health in recent years, and last year announced she was retiring from public life on doctor's orders.
However, with characteristic gusto, she has broken that order on several occasions.
Sir Denis' hereditary baronetcy will now pass down to his son, who will now become known, if he so chooses, as Sir Mark Thatcher, Bt.
This baronetcy was conferred on Mr Thatcher, as he then was, in 1990, shortly after his wife left 10 Downing Street. At the same time, she was awarded an Order of Merit.
The Conservative leader, Iain Duncan Smith, paid tribute, saying: "He was a truly great man.
"Without him frankly it would be impossible to imagine Mrs Thatcher being the success she was.
"He had a fantastic zest for life and was very humorous. And he was a success in his own right remember, as a businessman and a soldier."
Sir Denis underwent a five-hour heart bypass operation in January. He appeared to recover from this and after a spell of convalescence in a spa hotel, he travelled to South Africa to recuperate.
But, back in Britain, he felt unwell again two weeks ago and was admitted to the Royal Brompton Hospital and later transferred to the Lister Hospital where he died.
Lady Thatcher described him as the "golden thread" running through her life and often said that she could not have achieved what she did without his loyalty, love and support.
He was famed for what his daughter Carol described as his "copious" intake of gin, and would regularly hobnob with reporters when he accompanied his wife on official visits abroad.
Other high profile instances of Sir Denis' attitudes often leaked out, including referring to "fuzzy wuzzies in Brixton" and complaining that India was "high on the buggeration factor".
His son Mark had flown to London from his home in South Africa to be at his father's bedside.
His daughter Carol, who spends much of her time in Switzerland, is also in London.
The then plain Denis Thatcher had, in fact, retired in 1975, the year that his wife became leader of the Conservative party.
His wealth, and willingness to stay in the background of what was, paradoxically, a very radical partnership, proved very helpful to Mrs Thatcher. Indeed, he was once famously described as "a world-class chauvinist married to the supreme symbol of female equality".
Lord Heseltine, a member of Lady Thatcher's cabinet who later challenged her for the Tory leadership, paid tribute to Sir Denis.
"He always conducted himself in an exemplary way," the peer said. "He was a remarkable man who made a very considerable contribution."
Lord Heseltine told BBC News 24: "I saw enough of him to make a judgment about his incredible loyalty and strength.
"What I think of above all else is that he was just a very human man.
"He had his own views. He was unfailingly polite but firm and very courteous. But he was obviously an enormous strength to Mrs Thatcher.
"It was obvious to all of us, I think, who came across them that that was the case. I formed a great admiration for him."
"Denis Thatcher came out of those [the 'Dear Bill'] letters extraordinarily well because he was such a human guy and there was a great deal of humour there. I think that reflected the man," Lord Heseltine said.
In a statement, Theresa May, the Conservative party chairman, said: "He was a tower of strength to Margaret Thatcher during her historic premiership."
John Whittingdale, a Conservative MP who knew Sir Denis, said the Private Eye image was a "caricature - the real Sir Denis was a very sharp businessman. But he didn't mind the way he was portrayed, he almost played up to it".
He said he thought Sir Denis had been relieved when Mrs Thatcher finally stepped down as prime minister.
Lady Thatcher's former press secretary Sir Bernard Ingham told Sky News: "It's a very sad day because the former prime minister has lost her mainstay."
Sir Bernard said Sir Denis had been "a breath of fresh air" in Downing Street: "He had his own way of thinking and he didn't hesitate to express himself.
"He was a loyal, reliable husband and consort to the prime minister. He knew what his duty was, he knew how to discharge it and I think in many ways he was a model consort for a prime minister."
Sir Denis's death would be a great blow to Lady Thatcher, said Sir Bernard. "A great deal will have been lost in her life.
"She's not all that well in herself in terms of loss of memory. Therefore, I don't think she's going to find it easy to recover.
"She has, of course, in the past shown great powers of political recovery and you should never underestimate her but I think this is a very severe blow to her."
Neil Kinnock calles his former opponenet's spouse "a decent old cove. He was always very mellow - whether that was through drink, I don't know".
"He was also a first class rugby referee. He was a total Tory."