Philip served in the Royal Navy during the Second World War and saw active service against German, Italian and Japanese forces. The Greek prince's early life was also marked by upheaval - he escaped his home country as a baby by being hidden in a makeshift cot made from an orange box. So his words were filled with meaning when he told Princess Elizabeth in 1946 how his love for her made all his past struggle - and the horrors the world had just been through - seem trivial by comparison.

The letter, written in 1946 - a year before their wedding - was among several revealed in Philip Eade's 2011 book Young Prince Philip: His Turbulent Early Life. The Duke of Edinburgh, who has died aged 99, told the Princess how falling in love with her so 'completely' had made his personal troubles and even those of the world 'seem small and petty'

He wrote: 'To have been spared in the war and seen victory, to have been given the chance to rest and to re-adjust myself, to have fallen in love completely and unreservedly, makes all one's personal and even the world's troubles seem small and petty.' Three years earlier, Philip had spent Christmas at Windsor Castle. Princess Elizabeth was said to be animated in a way 'none of us had ever seen before', her governess, Marion Crawford, wrote.

Writing to her after seeing her again in July, Philip wrote of the 'simple enjoyment of family pleasures and amusements and the feeling that I am welcome to share them. 'I am afraid I am not capable of putting all this into the right words and I am certainly incapable of showing you the gratitude that I feel.' The same year, he apologised for the 'monumental cheek' of turning up to Buckingham Palace uninvited. 'Yet however contrite I feel, there is always a small voice that keeps saying "Nothing ventured, nothing gained",' he wrote. 'Well did I venture, and I gained a wonderful time.'

And in a letter to the Queen Mother two weeks after his wedding to Princess Elizabeth in November 1947, Philip expressed his vision for their time together. He said: 'Lilibet is the only thing in this world which is absolutely real to me and my ambition is to wield the two of us into a new combined existence that will not only be able to withstand the shocks directed at us but will also have a positive existence for the good... Cherish Lilibet? I wonder if that word is enough to express what is in me. Does one cherish one's sense of humour or one's musical ear or one's eyes?'

Philip told the Queen Mother in the year of her daughter's wedding to him how 'Lilibet' was the 'only thing in this world which is absolutely real to me.'

In the years since their wedding, both Philip and the Queen have spoken of each other with affection in public. Mischievous Philip is said to have joked to his wife on the day of her coronation in 1953 - when she was wearing the 17th century St Edward's Crown -'where did you get that hat?'

Princess Elizabeth and the Duke of Edinburgh, on honeymoon, photographed in the grounds of Broadlands looking at their wedding photographs, on November 23, 1947.

The Queen and the Duke shared an irreplaceable bond - united at key moments of history, witnessed from the viewpoint of a monarch and her consort.

In a 1997 toast during the couple's 50th wedding anniversary, he said: ' I think the main lesson that we have learned is that tolerance is the one essential ingredient of any happy marriage'. It may not be quite so important when things are going well, but it is absolutely vital when the going gets difficult. You can take it from me that the Queen has the quality of tolerance in abundance.'

She said on the same evening that Philip had been her 'strength and stay all these years. I, and his whole family, and this and many other countries, owe him a debt greater than he would ever claim, or we shall ever know,' she added. Nearly two decades later, at her 2012 Diamond Jubilee added: 'Prince Philip is, I believe, well-known for declining compliments of any kind. But throughout he has been a constant strength and guide.'