Queen Elizabeth has been left devastated by the death of her dog

A source said: "She does not like seeing her dogs suffer and she knows that sometimes putting a dog down is the kindest course of action - although that doesn't make her loss any less."

The dog was still being exercised and fed until the weekend, when her condition worsened and so the decision was made to call a vet.

The pooch has reportedly been buried in the grounds of the castle and a headstone will be erected to mark the spot, bearing Willow's name and the words "a faithful companion of the Queen".

Willow was a direct descendant of Susan, the first Corgi given to the queen on her 18th birthday, while the heritage line of the breed dates back to 1933 when the monarch's father, the late King George VI, who was Duke of York at the time, brought Dookie into the family.

Insiders said the queen has been hit "extremely hard" by the loss of her devoted companion.

A source told the Daily Mail newspaper: "She has mourned every one of her corgis over the years, but she has been more upset about Willow's death than any of them.

"It is probably because Willow was the last link to her parents and a pastime that goes back to her own childhood. It really does feel like the end of an era.

"Willow represents a significant thread running through the queen's life from her teenage years to her 90s.

"For many, many years she bred and raised corgis and to think that the last one has now gone is something of a milestone."

However, the queen does still have two dorgis - dachsund-corgi crosses - named Vulcan and Candy, and last year she agreed to adopt another corgi, Whisper, following the death of his owner, a former gamekeeper at Sandringham.

In 2015, it was revealed the monarch had stopped breeding Pembroke Welsh corgis because of fears that younger dogs may cause her to trip and hurt herself.

Willow's death came just 18 months after another corgi, Holly, passed away aged 13.

The queen has owned more than 30 corgis since 1945, with many of them direct descendants of her first dog Susan.

At one stage, she was said to have had as many as 13 of the animals.