David Cameron has admitted that he lived "in the shadow" of his older brother when he was a teenager.
The Prime Minister's comments come in the latest edition of the Big Issue, which he guest edited. They draw a parallel with the experience of Ed Miliband, who overhauled sibling David to win the Labour leadership last year.
Before that Ed - four years younger - followed David to Corpus Christi college at Oxford and then into parliament and government.
Alex Cameron, 47, a criminal lawyer who heads his chambers in London and is said to earn £1 million a year, was three years David's senior at Eton.
He went to Bristol University while David went to Oxford, before beginning his political career in the Conservative Party's research department.
In the publication's regular column, My Younger Self, the Prime Minister wrote: "I lived in the shadow of my older brother. He was three years older, went to the same school, and was a huge success, on the sports field and almost always lead actor in the school plays.
"It was great to have that kind of role model, and I was incredibly proud of him, but like many younger brothers you find yourself always a few steps behind.
"If I could give my younger self some advice, I'd say: don't worry about it; your life is not predetermined; you'll find your own feet in your own way. It was not until I left school that I felt I was breaking out of my brother's shadow and doing my own thing."
Mr Cameron conceded that, in contrast to his older brother, he "didn't always put the effort in" in sport and other subjects at school, and instead "just went through the motions, drifting along".
Mr Cameron praised his "strong family" and father Ian, who died last year.
He wrote: "My dad had an amazing ability to always look on the bright side of life. He was disabled. He had short legs, not enough toes and no heels, yet he did everything with us - tennis, swimming, holidays, and was always the most tremendous fun.
"Because I grew up with him, I am not sure I even realised how amazing he was - if I was 16 again, I'd tell him."
He added: "A lot has been written about my background, but the great privilege of my upbringing wasn't just the wealth, it was the warmth. We all got on, we were all there for each other, there was so much love and support."
The Prime Minister said his political philosophy was formed by his travels round Eastern Europe after leaving school - including a trip to the former Soviet Union.
"I will never forget the greyness of life under Communism, the lack of choice, freedom and expression," he added.
"I began to develop a political consciousness, a sense of what was right and wrong. In particular, the importance of freedom and the state being there to serve people, not be their master."