Prince Philip was notably absent from the royal family's annual Christmas service, prompting speculation he was suffering from ill health.
Although the Palace says he was doing "absolutely fine", GLOBE reports "his end is very near."
The Duke of Edinburgh was last seen publicly at Princess Eugenie’s wedding in October.
“At the time, he was holloweyed and looked like the walking dead,” added the courtier.
“His situation has gotten much worse since then. He struggles to think and talk, and has trouble recognising anyone, even his wife.
According to the source, Philip is "stubborn" and vows to hang on long enough to meet Harry and Meghan's baby.
"He is demanding his doctors help him live long enough to see Prince Harry’s first child with Meghan Markle in the spring," they added.
“I’m told doctors are using a cocktail of powerful drugs — stimulants and sedatives — to help him move around when he wants and rest when he needs to."