The Queen's friend and former Royal Studs manager died on the day of Prince Philip's funeral, four days short of his 87th birthday.
Sir Michael Oswald became manager of the Royal Studs in 1970, a role he held for 28 years, according to the Racing Post. He was also the Queen Mother's racing manager until her death in 2002.
In 2003 he became National Hunt Racing Adviser to the Queen, and was often seen with her and other members of the Royal Family at races and derby days.
Sir Michael was married to Lady Angela Cecil, who had her own ties to the Royal Family, and was one of the Queen Mother's women of the bedchamber.
Sir Michael and Lady Angela married on 21 April 1958 - his birthday and the Queen's. Lady Angela joked that the couple received a gun salute every year for their anniversary.
His death on 17 April will bring more sadness to a royal household which remains in mourning after the death of Prince Philip on 9 April.
The Queen officially stays in royal mourning until 22 April, the day after her 95th birthday.
Her husband's scaled back funeral was held on 17 April, in Windsor Castle.
Lady Angela told the Post: "He always said he had the most wonderful job anybody could ever have had and that for all his working life he was simply doing what he would have done had he been a rich man who didn't have to work."
Racehorse trainer Nicky Henderson told the Post: "He was very close to the Queen and the Queen Mother and loved his role with their horses. He would go absolutely anywhere to watch them run, even in the last few years when he was in his eighties.
"He lived in Norfolk, but would happily drive all the way to Exeter. The Queen once said to me that we had to stop Michael travelling all over the country. I did very respectfully point out she was the only one who could do that."
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He also recalled: "A few years ago we won the EBF Final at Sandown for the Queen with Close Touch. The race was sponsored by Paddy Power and for the trophies Paddy Power had made three bronze sets of pants.
"They were laid out on the table ready for the presentations, when they suddenly realised that giving the Queen a pair of bronze pants might be deemed inappropriate, so they removed that particular trophy and replaced it with a vase.
"Sir Michael made very clear he wanted the pants not the vase and announced he would be driving them straight to Windsor Castle. The following morning I spoke with the Queen, who said the Duke of Edinburgh had been highly entertained by the prize."
Sir Michael was born in Walton-on-Thames in Surrey in 1934, and went to Eton, reported The Times.
He served in the Scots Guard in the Korean War, but did not frequently speak of his military service.
When he returned from war he found success in the equestrian world and went on to work for the Royal Family at the estate in Sandringham.
He insisted the Queen did not bet on her horses, saying it was the joy of owning the horse which she loved.
He said: "I have never known the Queen ever to have had a bet herself.
"She has a sweepstake, about a pound an entry, in the box before the Derby. But that’s not betting on horses as such. For her the passion is having a horse that wins. There is nothing she doesn’t know about a horse’s pedigree."
He once said of his job: "Having been paid to do what I would have done for fun had I been a rich man, I must be the luckiest of all chaps, because there was never a better and more knowledgeable owner to answer to."
Sir Michael died after a long illness on 17 April, at the age of 86.
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