Princess Diana and the Queen's relationship was tested on many occasions in the years they knew each other.

But one particular incident that is said to have raised eyebrows involved Diana and Her Majesty, 93, during one Christmas lunch at Sandringham.

Diana, who died aged 36 in 1997, reportedly asked a 'brutal question' which left the rest of the Royal Family  - including Prince Charles - 'looking at her as if she were mad.'

It was while around the dinner table, Diana reportedly broached the subject of the Royal Family and a federal Europe.

According to royal biographer Andrew Morton, he claims Diana asked whether the royals would be relevant.

Diana's question is said to have shocked the Royal Family

Writing in his book Diana: Her True Story, Mr Morton wrote: “The Queen, Prince Charles and the rest of the royal family looked at her as if she were mad and continued with their debate on who shot the last pheasant of the day, a discussion which occupied the rest of the evening.

“As a friend says: ‘She finds the monarchy claustrophobic and completely outdated with no relevance to today’s life and problems.

“She feels that it is a crumbling institution and believes that the family won’t know what has hit them in a few years’ time unless it changes too’.”

Meanwhile, it was after Diana's death in Paris that it emerged she had helped with the blisteringly honest book Diana: Her True Story.

The mum-of-two collaborated with royal biographer Mr Morton and got her friends and family to talk candidly about her troubled marriage with Charles.

The pair had their differences

Details of life inside the House of Windsor and even her relationship with the Queen were revealed.

The Queen was allegedly "stunned" by what was written and that her daughter-in-law would air her "dirty linen in such a way."

But Diana admitted she felt "desperate" and as if she was "at the end of her tether."

However despite clashing over their differences, the Queen described Diana as "an exceptional and gifted human being."

In a public tribute to Diana days after her death, Her Majesty said: "Since last Sunday's dreadful news we have seen, throughout Britain and around the world, an overwhelming expression of sadness at Diana's death.

Diana wasn't afraid of speaking her mind

"We have all been trying in our different ways to cope. It is not easy to express a sense of loss, since the initial shock is often succeeded by a mixture of other feelings: disbelief, incomprehension, anger - and concern for those who remain.

"We have all felt those emotions in these last few days. So what I say to you now, as your Queen and as a grandmother, I say from my heart.

"First, I want to pay tribute to Diana myself. She was an exceptional and gifted human being. In good times and bad, she never lost her capacity to smile and laugh, nor to inspire others with her warmth and kindness.

"I admired and respected her - for her energy and commitment to others, and especially for her devotion to her two boys.

"This week at Balmoral, we have all been trying to help William and Harry come to terms with the devastating loss that they and the rest of us have suffered.

The Queen said Diana inspired others with her warmth and kindness
 

"No-one who knew Diana will ever forget her. Millions of others who never met her, but felt they knew her, will remember her.

"I for one believe that there are lessons to be drawn from her life and from the extraordinary and moving reaction to her death.

"I share in your determination to cherish her memory.

"This is also an opportunity for me, on behalf of my family, and especially Prince Charles and William and Harry, to thank all of you who have brought flowers, sent messages, and paid your respects in so many ways to a remarkable person.

"These acts of kindness have been a huge source of help and comfort.

The trio pictured at Buckingham Palace on March 7, 1981

"Our thoughts are also with Diana's family and the families of those who died with her. I know that they too have drawn strength from what has happened since last weekend, as they seek to heal their sorrow and then to face the future without a loved one.

"I hope that tomorrow we can all, wherever we are, join in expressing our grief at Diana's loss, and gratitude for her all-too-short life.

"It is a chance to show to the whole world the British nation united in grief and respect.

"May those who died rest in peace and may we, each and every one of us, thank God for someone who made many, many people happy."