On Monday, a week after he stunned Singapore when he nearly fainted while giving an annual policy speech live, Prime Minister Lee Hsien Loong is due back at work. Following medical leave of seven days, it may be business as usual for him. But the incident has drawn attention to the unusual level of uncertainty around leadership succession.

Regardless of the state of the premier’s health, Singapore’s ruling party likes to identify successors many years in advance. Lee himself broached the subject when he went back on stage to resume his address. He gave himself the deadline of stepping down and handing over power to his successor “soon after” the next general election, which must be held before January 2021.

Just who that successor will be, however, is unclear.

In past leadership changeovers, successors were identified early and underwent lengthy understudy.

Lee was deputy prime minister for 14 years before he took over in 2004 from his predecessor Goh Chok Tong. Goh was deputy premier for five years before he became PM in 1990, taking over the reins from Lee’s father, the late founding prime minister Lee Kuan Yew. There was an age gap of at least 10 years between successive premiers.

In contrast, the current deputy prime ministers, Teo Chee Hean and Tharman Shanmugaratnam, are just three and six years younger than Lee Hsien Loong’s 64 years. For this reason, neither man is seen as satisfying the need for leadership rejuvenation.

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Lee has twice survived cancer. He shocked everyone that Sunday evening when he suddenly stopped speaking, slouched, gazed sideways and held on to the rostrum. As cameras turned away from him, he was whisked away by his security detail and three cabinet ministers in the audience who rushed onstage.

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The National Day Rally, which involves Malay, Chinese and English speeches by the prime minister, was suspended for 82 minutes as Lee received medical attention.

He re-emerged looking energised, and waved to the cheering crowd before completing an abbreviated version of his speech.

For several decades, succession in Singapore has been touted as a non-event – carefully orchestrated political ballets devoid of the uncertainty that usually accompanies transitions in many of its Southeast Asian neighbours.

“What happened makes it more important that I talk about it now,” Lee quipped, after acknowledging that he “gave everybody a scare”.

Some political experts voiced concern at the episode, which was downplayed by Lee’s office as a result of “prolonged standing, exhaustion and dehydration”.

Bridget Welsh, a Singapore politics expert at the National Taiwan University, said “events at the rally reinforced Singapore’s leadership vulnerability”.

And Garry Rodan, professor of Southeast Asian politics at Murdoch University in Australia, said: “The pace and nature of structural pressures on Singapore policymakers has rapidly mounted, rendering succession planning more urgent than in the past”.

In his speech after the forced intermission, Lee stressed he was focused on ensuring a smooth leadership transition.

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“We have now got the core team for the next generation in cabinet. But ministers or not, all of us are mortal,” said Lee, a former army brigadier general who entered politics in 1984.

“Building up leadership and preparing for succession is one of my top priorities. Nothing that has happened has changed my timetable or my resolve to press on with the succession,” Lee said.

He announced that Finance Minister Heng Swee Keat, seen by some as a potential future premier, was ready to return to work after suffering a stroke in May.

“Progressively Swee Keat will come back to work,” Lee said, adding that Lawrence Wong, minister for national development, would assist 54-year-old Heng in the finance ministry.

“In the next GE [general election] we will reinforce the team again. And soon after the next GE, my successor must be ready to take over from me,” Lee said, before using a Chinese phrase roughly translatable as time waits for no one.

Singapore’s next general election must be held by January 2021.

Lee’s People’s Action Party (PAP), which has ruled the city-state since 1959, staged a crushing victory in the last general election, in September last year, winning 69.9 per cent of the popular vote and 83 out of 89 parliamentary seats. The polls were held six months after the death of Lee Kuan Yew at age 91.

Longtime Singapore politics observer Eugene Tan said Lee’s comments on when he wanted to step down offered the “clearest timeline yet on succession”.

“We are looking at about six years away. There is some concern about whether there is a clear successor,” Tan said.

The lead time from when the next premier is named to when he takes over will be far shorter compared to previous leadership changes: Lee’s 14 years and his predecessor Goh’s five years.

In both instances, the new prime minister was picked by his peers in cabinet.

While Lee’s deputies Teo and Tharman are seen as out of the running, talk has resurfaced about Tharman being a worthy successor given his widely-regarded credentials. Online, at least one prominent Singaporean cited his name as contributing to a “future we can all believe in”.

Rodan said: “Arguably nobody is better equipped to succeed Prime Minister Lee in this exercise than Tharman Shanmugaratnam. He has demonstrated strengths across economic and social policy combined with an authoritative but unpretentious political style capable of engaging critics as well as supporters.”

But the more likely scenario would be to stick to the script of finding a younger person in the interest of renewal.

“The decisive moment will be when someone from the fourth generation of leaders is named deputy prime minister,” Tan said.

“Anyone who thought that Heng Swee Keat would be the successor was forced to re-evaluate because of his recent health situation,” said Tan, an associate law professor at the Singapore Management University.

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Finance Minister Heng, a former central bank chief, led several key government committees before he suffered a stroke during a cabinet meeting in May.

He returned to work on Monday, after spending six weeks in hospital and further time recuperating at home.

Heng’s health scare came 15 months after Lee himself underwent a successful surgery for prostate cancer. Lee subsequently said doctors gave him the all clear following the surgery.

Lee was diagnosed with lymphoma in 1992, for which he underwent chemotherapy and is now in remission.

Political analysts say Chan Chun Sing, the leader of the PAP-aligned National Trades Union Congress and the ruling’s party’s parliamentary whip, is also a frontrunner for the premiership. The former army chief is a cabinet minister without portfolio.

Another contender is acting Education Minister Ong Ye Kung, a former top civil servant who served as principal private secretary to Lee.

“I would put Minister Heng and Minister Chan as the frontrunners with Ong Ye Kung as the dark horse,” said Tan.

Chong Ja Ian, an assistant professor of political science at the National University of Singapore (NUS), said the future prime minister would need to display an ability to handle greater political and social pluralism in the city-state, where virtual one-party rule has been the norm since independence in 1965.

“There has to be constant calibration between majoritarian impulses and the protection of minority rights,” Chong said.

“Managing these tensions are not easy and there are no off-the-shelf technical fixes,” Chong said.

The current prime minister has faced increasingly strident criticism from activists who accuse his government of curbing civil liberties in recent years.

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Earlier this month, his younger sister Dr Lee Wei Ling surprised Singaporeans by joining the fray, branding the current government as “authoritarian” in a Facebook post criticising a new legislation that codified contempt of court law.

The younger Lee, the second of Lee Kuan Yew’s three children, said the law was “an attempt to muzzle public opinion”. In April, Dr Lee used the same platform to accuse her brother of abusing his powers, a charge he denied.

With the increasingly boisterous political landscape, the new Singapore leader will have to master the art of working “with difference and disagreement in a non-hierarchical setting”, Chong, the NUS professor, said.

“Continued debate, discussion and disagreement are the only constants, and the only higher authority are citizens and perhaps shared political values,” Chong said.