“I would have too short a runway should I become the next prime minister then,” Heng said in his letter. Heng is 60 while prime minister Lee Hsien Loong is 69.

“We need a leader who will not only rebuild Singapore post-Covid-19, but also lead the next phase of our nation-building effort,” he added.

He is also set to give up his portfolio as finance minister, local media reported.

He will continue serving as deputy prime minister but will step aside as leader of the ruling People’s Action Party’s (PAP) fourth-generation team, a term used to describe the group of ruling party politicians seen as future leaders.

“This unexpected turn of events is a setback for our succession planning. We recognise that Singaporeans will be concerned.

“We seek your support and understanding, as we choose another leader for the team,” the 4G leaders said in a separate letter.

Heng, who steered the Singapore central bank through the global financial crisis, has doled out billions of dollars in stimulus to help carry the country through the economic downturn caused by the Covid-19 pandemic.

But questions were also raised about his health after he suffered a stroke and collapsed during a Cabinet meeting in 2016.