Sir Denis Thatcher, the husband of former Prime Minister Margaret Thatcher, has died, a spokesman for the family has announced.
Baroness Thatcher said she could not have done without Sir Denis
The 88-year-old, who had heart surgery in January this year, returned to hospital after feeling unwell two weeks ago.
He died at the Lister Hospital in London on Thursday morning.
A spokesman for the Thatcher family 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."
Baroness Thatcher and their children Mark and Carol were at Sir Denis' bedside when he died.
The union flag at Conservative Central Office in London was lowered on Thursday in a mark of respect for Sir Denis.
Sir Denis had originally been at the Royal Brompton Hospital, where doctors had said they wanted him to have more tests when he was readmitted to hospital as a "precautionary measure".
He was later transferred to the Lister Hospital.
He was said to have been in good spirits and had received several visits from Lady Thatcher.
Although he had enjoyed success as a millionaire businessman, Sir Denis' public fame owed most to his marriage to Margaret Thatcher.
His motto in that role was the prime ministers' consorts should be "always present, never there".
Sir Bernard Ingham, Baroness Thatcher's former press secretary said: "I am extremely sad that he has died. I have very many fond memories of him as the prime minister's consort and husband.
"He was extremely loyal and supportive in that position. He was in fact a manifest role model for any prime minister's consort."
Lady Thatcher said of her husband in her autobiography, "The Downing Street Years": "I could never have been prime minister for more that eleven years without Denis by my side.
"He was a fund of shrewd advice and penetrating comment. And he very sensibly saved these for me rather than the outside world."
Conservative leader Iain Duncan Smith said he was devastated by the death of "one of the most decent, determined and kind people that we have known".
"In a world that so often seems to have lost its manners, Denis Thatcher represented so much of what was best in the wartime generation."
Mr Duncan Smith said Sir Denis had played a "vital role" in his wife's success, as well as being a successful businessman and brave soldier in his own right.
Sir Denis was in buoyant mood when leaving hospital earlier this year
Tory MP John Whittingdale, who used to be Baroness Thatcher's political secretary, described Sir Denis and immensely courteous and very generous.
"Denis was held in enormous respect and affection through the whole country, not matter what people's political views," he said.
"He made a huge contribution to Mrs Thatcher's political career. He was behind the scenes all the time giving her support and I think she and her family will miss him dreadfully."
At a news conference with Russian President Vladimir Putin, Tony Blair said: "Sir Denis was a kind and generous-hearted man, a real gentleman who had many friends here and abroad.
"I know how much he meant to Margaret Thatcher and to the whole family and I know whatever part of the political spectrum we are on, we would like to send our deepest condolences and sympathy to the Thatcher family."
There was a tribute from former Daily Telegraph editor Bill Deedes, the recipient of Sir Denis' fictional Dear Bill letters in Private Eye.
Lord Deedes, a close friend of Sir Denis, said: "There were hidden depths to Denis Thatcher. He was very self-effacing.
"He kept himself out of the limelight and he concealed his virtues for his friends as well as everyone else."
Lord Deedes said Sir Denis had been a "bulwark" to Baroness Thatcher and was able to provide a calmer outlook at times.
Sir Denis left hospital 11 days after his heart by-pass in January, with doctors calling the operation a "complete success".
At the time his daughter put his quick recovery down to "copious gin drinking".
Last year, Baroness Thatcher turned down an invitation to visit the Falklands, saying the trip would have been "too gruelling" for her husband.
Sir Denis is said to have been the key influence in persuading his wife to bow out of public speaking after she suffered a series of slight strokes.
She has defied doctors, however, by attending a number of functions.
Baroness Thatcher said she could not have made it to the top without him.
They were married for more than 50 years, including 11 at Downing Street.