Chief Executive Leung Chun-ying has made a surprise announcement that he will not seek re-election due to family reasons.
"If I run my family will suffer intolerable stress due to my electioneering ... I must protect them," he told a press briefing.
He told reporters his decision was not due to the central government not endorsing him.
"[Beijing] has always supported me and said I have done a good job," Leung said.
Asked if the decision was because of his daughter Leung Chai-yan's condition, he said he did not want to delve too much into his personal life, but said: "As a husband and father I have a responsibility ... My daughter has only one father, and my wife has only one husband."
Chief Executive Leung Chun-ying meets the media on Friday. Photo: Felix Wong
On Thursday night, he was seen at Prince of Wales Hospital in Sha Tin, where Chai-yan was rumoured to be receiving treatment.
He added that his past political career had already impacted his family.
Leung had repeatedly told media "there was no message" yet when asked whether he would seek a second term in March.
During a trip to an Asia-Pacific Economic Cooperation meeting in Peru last month, Leung told the media that President Xi Jinping had approved of his work in office, but he later said people should not "over-interpret" the remarks as being any endorsement of further political ambitions.
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Leung assumed office as Hong Kong's fourth chief executive in 2012, succeeding Donald Tsang Yam-kuen. Leung was a member of the Executive Council from 1997 to 2011, prior to taking over the top spot.
If I run my family will suffer intolerable stress due to my electioneering ... I must protect them.
Leung refused to comment on who he would support to be the next chief executive, with Financial Secretary John Tsang Chun-wah or lawmaker Regina Ip Lau Suk-yee both widely expected to run – but said he would back anyone who managed to win majority support in the Election Committee.
Chinese-language newspaper Apple Daily reported earlier this week that Chai-yan had been hospitalised for a month at Prince of Wales Hospital.
Leung Chai-yan (centre) is said to have suffered from emotional problems over the past two years. Photo: SCMP Pictures
On Thursday, the chief executive was seen in the Main Clinical Block and Trauma Centre of the hospital, but he refused to comment on his daughter's situation.
Chai-yan is said to have suffered from emotional problems over the past two years.
In 2014, she posted a photograph on Facebook that appeared to show her lying in a bathtub with a slashed wrist and the water tinged red. "Will I bleed to death?" she wrote.
In March last year she wrote on social media that she had "officially" left home, a day after claiming online that she had been in a violent row with her mother.
Jeffrey Lam Kin-fung, a legislator and member of the Executive Council, confirmed that Leung's daughter was in hospital but added he was surprised by the chief executive's decision.
"I'm a bit surprised," he said. "I know his daughter has been hospitalised for a while. I think his care for his family should be respected and understood. Family is important to everyone."
He praised Leung's contributions to Hong Kong over the past few years, especially on housing.
The Hong Kong and Macau Affairs Office of the State Council issued a statement saying it respected Leung's decision. It said he had "staunchly implemented the one country, two systems formula and the Basic Law", and had made important contributions in defending the sovereignty of the state, security, development and Hong Kong's stability. It added that the central government had always fully recognised and rated his work highly.