Princess Diana
The Queen's relationship with Princess Diana was strained. Reuters

When the Queen found out that Princess Diana had been in a car accident in Paris, she allegedly said "someone must have greased the brakes". This claim is made in a new book by royal biographer Ingrid Seward, according to the Daily Mail.

Diana was killed in a high-speed car crash in August 1997, leading to a period of national mourning for the popular royal.

The Royal Family, however, were criticised for apparently reacting in an unemotional manner.

Seward told the Daily Mail: "At first it was thought that, though the car crash in the Point de l'Alma was serious, Diana had not been killed.

"[And] according to one witness present when the Queen heard the initial news, she mused out aloud: 'Someone must have greased the brakes.'"

The offhand comment revealed the complexity of the relationship between the Queen and Diana, Seward added.

The book reveals that the Queen was initially fond of her future daughter-in-law prior to the wedding, even describing Diana as "one of us".

Their relationship began to suffer as Diana's anxiety increased due to the expectation of being a royal.

Diana's wedding to Charles at St Paul's in July 1981 was on a scale never seen before, not even for the Queen's Coronation, Seward told the Daily Mail.

Seward said: "On the day itself, a vast crowd gathered in the Mall to see Charles and his bride appear on the balcony. Listening to the roars, Diana said to her husband: 'They want us to kiss.' They did, and the moment was shared by a worldwide TV audience of 700 million."

The media attention, however, was unceasing and Diana struggled to cope with the pressure of being a royal, leading to their relationship being strained.

When Diana was killed in the car crash in Paris, Buckingham Palace did not comment publically for days and refused to fly a flag at half-mast, according to the Daily Mail.

This has some fuelled conspiracy theories that the Queen despised Diana or even played some part in her death.

But the decision not to comment was made because the family was in shock and the flag over the Palace was not flown at half-mast because the Queen was not in residence, according Seward's book.