Keir Starmer said his decision to resign as UK Prime Minister and Labour leader was an "intensely personal" one, reached during a family stay at Chequers.
He vowed to support his successor without interfering, while continuing as the MP for his London constituency.
Starmer defended his foreign policy focus, saying future prime ministers cannot afford to spend less time on international affairs amid growing global instability.
Outgoing British Prime Minister Sir Keir Starmer has described his decision to resign as Labour Party leader and Prime Minister as an "intensely personal" one, saying it followed careful reflection with his family on what would be best for himself, the government and the country.
Speaking to the BBC in his first interview since announcing his resignation on June 22, Starmer said the final decision came during a two-day family retreat at Chequers, the official country residence of the British Prime Minister, where he was joined by his wife, Victoria, and their children.
"I grappled with what was the best thing to do for me, for the country, for the government. In the end it became an intensely personal decision," Starmer said. "Vic and I were away with the kids. We just spent two days together as a family and that's when I came to my final decision."
The 63-year-old Labour leader said ending a political career was never an easy choice.
"Taking the decision that your political career is over, it is an intensely personal matter, or at least it was for me," he said.
Starmer confirmed that although he would be leaving Downing Street, he intends to continue serving as the Member of Parliament for his central London constituency until at least the next general election. He also pledged not to undermine his successor by offering unsolicited advice.
"I will be keeping my mouth shut, rather than giving constant advice to my successor about what they should be doing," he said.
His resignation has triggered a Labour leadership contest, with former Greater Manchester Mayor Andy Burnham widely seen as the frontrunner to become both Labour leader and Britain's next Prime Minister. Despite the speculation, Starmer declined to endorse any candidate and instead promised to back whoever succeeds him.
"I've never had any personal animosity towards Andy Burnham," Starmer said. "I'll do everything I possibly can to make sure the next government succeeds."
Reflecting on his tenure, Starmer argued that one of his greatest achievements was rebuilding the Labour Party after years of political setbacks. He described the party as "politically, financially and morally bankrupt" when he became leader and said restoring its credibility required "hard and bloody work."
However, he acknowledged that his resignation became inevitable after Labour MPs no longer believed he was the right person to lead the party into the next general election.
Starmer also defended the amount of time he devoted to international diplomacy while in office. Rejecting suggestions that future prime ministers should focus more on domestic issues, he argued that global and domestic challenges are inseparable.
"We're in a more dangerous and volatile world than we've been in for probably most of my lifetime," he said. "That's not just a phrase, that's reality."
Asked whether his successor could spend less time on foreign affairs, Starmer replied, "No, I don't think it is possible."
"There's often this discussion—what's the right balance between dealing with international affairs and dealing with domestic affairs? They're one and the same thing," he added.
Starmer's departure marks the end of a significant chapter for the Labour Party, which he led back to government after years in opposition. As the party prepares to elect a new leader, Britain now enters a period of political transition, with the incoming prime minister expected to confront persistent economic pressures, domestic policy challenges and an increasingly uncertain international environment.
