Former UK prime minister Boris Johnson has claimed that Queen Elizabeth II was sick with "bone cancer" in the final years of her life. 

It is a highly unusual revelation, as the queen's health was closely guarded information during her reign and meetings between the reigning monarch and British prime minister are usually confidential. 

So what did he say?

In an excerpt obtained first from the UK Daily Mail from his soon-to-be released memoir Johnson recalled a visit to see the queen at Balmoral castle on September 6, 2022 just two days before she died. 

Johnson was meeting the queen for the final time in his role as prime minister before he left office. 

"Edward Young, her private secretary, tried to prepare me," he wrote. 

"I had known for a year or more that she had a form of bone cancer, and her doctors were worried that at any time she could enter a sharp decline."

He said he was told the queen had "gone down quite a bit over the summer" and then the footman showed her into her majesty's drawing room. 

"She seemed pale and more stooped, and she had dark bruising on her hands and wrists, probably from drips or injections," Johnson wrote.

He said however that her mind was "completely unimpaired by her illness and from time to time in our ­conversation she still flashed that great white smile in its sudden mood-lifting beauty."

There has been no suggestion or official statement by the palace or any members of the royal family that the queen had cancer. 

"It is unusual for Boris Johnson to speak about the queen’s health but the former British prime minister isn’t the first to talk about it," royal commentator Juliet Rieden explained.

"Royal biographer and TV presenter Gyles Brandreth talked about rumours that the late queen had a rare form of myeloma in his book Elizabeth: An Intimate Portrait."

In the 2022 biography, Brandreth wrote: "I had heard that the Queen had a form of myeloma — bone marrow cancer".

"The most common symptom of myeloma is bone pain, especially in the pelvis and lower back, and multiple myeloma is a disease that often affects the elderly.

"Currently, there is no known cure, but treatment — including medicines to help regulate the immune system and drugs that help prevent the weakening of the bones — can reduce the severity of its symptoms and extend the patient's survival by months or two to three years."

At the time the queen's former spokesperson Dickie Arbiter told British publication Newsweek that he didn't "think anyone knows what it was". 

What was the queen's official cause of death?

On the queen's death certificate her cause of death was listed as 'old age' without any further details.

The certificate records the queen died at 15:10 on September 8, in Balmoral Castle, at the age of 96.

Did the palace mention any illness in the lead up to her death?

In the year before the queen died the only mention of her health by the palace was when it said she suffered from "episodic mobility issues" and a bout of COVID in early 2022.

She cancelled several events in 2021 and then kept a low profile in the last few months of the year due to the threat of COVID. 

On February 20, 2022, she returned a positive test for COVID but only came down with mild symptoms.

The queen performed only light duties following her bout with COVID, and missed public events such as Commonwealth Day services and the opening of the UK parliament.

The weekend before she died, it was announced the queen would miss the Braemar Royal Highland Gathering, a popular part of the Highland Games, which she previously attended almost every year.

On the Tuesday she received Britain's new prime minister Liz Truss, in Scotland, rather than returning to London to formally ask her to form a government as was tradition.

The day before she died she postponed a Privy Council meeting on the advice of doctors, who told her to rest.

Buckingham Palace announced her death on Thursday UK time, saying the queen died peacefully at Balmoral Castle that afternoon.

"Buckingham Palace is not commenting on Boris Johnson's revelations in his memoir that claim the queen had cancer and that he knew 18 months before she died," Rieden said.

"Johnson claims he was told in his capacity as prime minister to prepare him for her decline in health and if true this information would have been confidential so it does seem inappropriate for him to reveal that private information now in order to sell his biography.

"While they would not comment publicly I imagine the royal family would rather the queen’s private health issues remained private."

What else is in this book?

The prime minister's memoir, due for release in Australia on October 16 will also discuss his leadership through COVID, Brexit and the controversies that saw the end of his tenure as British leader. 

He also said he ordered military chiefs to plan a raid on a Dutch factory in March 2021 to secure 5 million COVID vaccines that the European Union had threatened to bar from being exported to Britain.

Johnson said the deputy chief of Britain's defence staff at the time, Lieutenant General Doug Chalmers, had told him a raid using small boats to cross the Channel and navigate Dutch canals would be possible — but warned him of diplomatic repercussions.

"I secretly agreed with what they all thought but did not want to say aloud: that the whole thing was nuts," Johnson said in an extract from his memoirs that was published in Saturday's Daily Mail newspaper.

In a press release Harper Collins Australia writes the book is "Written in his inimitable style, it is honest, unrestrained and deeply revealing about the politician who has dominated our times."