Billy Vunipola has defended his decision to withdraw from the summer’s British & Irish Lions tour because of a shoulder injury, saying that “otherwise I’d be letting people down” and insisting “there are more important things in life”.

Writing in his new book, Wrecking Ball, Vunipola explains it was a long-standing shoulder injury that forced him to pull out of the touring squad, just hours before he was due to join a Lions training camp.

Vunipola withdrew from the Lions squad the day after Saracens’ last game of the season against Exeter in May because of his shoulder injury. He was due to report for a training camp in Dublin but opted against travelling to Ireland, despite Warren Gatland asking him to do so.

Instead, Vunipola underwent surgery and is expected to make his comeback ahead of schedule for Saracens against Newcastle in Philadelphia on Saturday. At the time it was considered a huge blow to the Lions’ chances in New Zealand but the squad, that featured Billy’s brother Mako, proceeded to claim an unlikely drawn series against the All Blacks. The 24‑year‑old No8, who would have been a likely starter on the tour, says he has no doubts over the decision and that it was not influenced by anybody else.

“My conscience told me that I shouldn’t go on the tour because otherwise I’d be letting people down,” Vunipola writes in the book. “I’m not saying that from a selfish point of view, as if I wanted to go to New Zealand and be the star. But I didn’t think I’d be able to help the team in a positive way on the pitch. What was the point in touring if I was going to be a hindrance?

“Having learned from so many experiences, I’m now at a stage where I’m confident in what I stand for and what I believe in, so I have no doubts that I made the right decision.

“And it was my decision. It was important for me to show that I was mature enough to make the call myself and not wait for somebody else to do it for me. Sometimes my body just can’t keep up with all the demands I place on it, and that’s just the nature of the beast. The injury was something that happened a long time before the tour. I tried to tough it out but the timing wasn’t right for me on that occasion.”

While Vunipola is yet to feature for Saracens this season, he did take part in England’s pre-season training camp in Teddington last month. Asked about the No8 during the camp, England’s head coach, Eddie Jones, described Vunipola as having “the potential to be the best player in the world”. Next Wednesday marks exactly two years until the 2019 World Cup and Vunipola has said that the prospect of taking part in the tournament in Japan has softened his Lions blow.

“What helped me out massively was the realisation, which I’d had a couple of years before, that I should never define myself by rugby – there really are more important things in life,” he writes.

“When I die, I don’t want to be remembered for whether I went on a Lions tour or not. I’d much rather be known for whether I was a kind or honest person. To me, that’s way more important.

“There’s an absolute stack of rugby still on the horizon in any case, with a World Cup around the corner and loads more Six Nations and Saracens action to come. So I can’t look at it in any other way than I’m in a privileged position.”