Samantha Cameron has revealed how the death of her young son, Ivan, "overshadows everything".
The fashion entrepreneur said his premature death in 2009, the year before David Cameron entered No 10, was of such magnitude that it made everything else "irrelevant" and "meaningless".
Their first child, Ivan was born with a severe form of epilepsy and cerebral palsy and died shortly before his seventh birthday.
In an emotional interview for The Times she described how they had to cope with regular "life or death situations" throughout the youngster's life, yet it came as a "huge shock".
Mrs Cameron said she sought strength from her faith and found hosting charity events at Downing Street cathartic, although she would often be brought to tears when meeting children and parents in similar situations.
Meanwhile the experience coloured her husband's politics, she told the newspaper.
"It's the biggest thing in my life. Being the prime minister's wife was just a role," she said.
"Ive dying is such a massive thing that everything else is irrelevant. It just overshadows everything. What goes on in the outside world becomes meaningless.
"Like anyone else in my situation, I just kept going. You have to deal with it, because you have no choice."
Mrs Cameron said the couple relied on routine so they did not "fall apart completely" and they still visit Ivan's grave "very often".
When Mr Cameron announced his resignation after losing the referendum in June he appeared on the steps of Downing Street flanked by his wife and Ivan's siblings, Nancy, Florence and Arthur, who had grown up the flat above No 11.
In a way their brother's death had helped her prepare for the "parallel universe" of life in Westminster, Mrs Cameron explained.
"I think it changed Dave's politics. It made him understanding, although he couldn't be too subjective," she said.
The family now spend their time between homes in Notting Hill, west London, and Oxfordshire, where Mr Cameron served as MP for Witney for 15 years.
Mrs Cameron, who worked for British leather brand Smythson while he was PM, has now launched her own women's wear label, Cefinn.
PA
