The labour movement will get a new president on Thursday in a move to refresh its leadership.
The incumbent president of the National Trades Union Congress (NTUC), Ms Diana Chia, is not seeking re-election at its National Delegates Conference, which starts on Tuesday.
The 59-year-old is NTUC's first woman president. She was first elected to the post in 2011.
Her retirement, together with two other top grassroots unionists, was announced by Prime Minister Lee Hsien Loong on Monday night at a dinner to mark the conference's opening.
Besides Ms Chia, secretary for financial affairs Lim Kuang Beng, 59, and vice-president Nora Kang, 60, are also not seeking re-election to the NTUC's central committee, its highest decision-making body.
On her decision to step down, Ms Chia, who is a nurse, said: "As I cannot serve the full term, I think I should leave and let the new leadership take over."
In 2011, the NTUC implemented a self-imposed leadership renewal rule that requires top union leaders to voluntarily step aside to make way for new blood when they turn 62.
If the trio had stood and won at the union elections, their age would have prevented them from being able to complete the full four-year term, which ends in 2019.
Elections to the central committee are held every four years.
"They have been effective union leaders," said PM Lee of the trio.
"They understand workers' concerns, they know the national interest, they have influenced the Government and helped the Government to implement policies which benefit workers."
PM Lee said he had worked closely with them. He added: "We speak candidly with one another, we know how each other thinks, we can rely on each other to get things done."
Union sources said the frontrunners for the top NTUC president post are two incumbent vice-presidents: Mr K. Karthikeyan, 56, and Mr Edwin Lye, 45.
Mr Karthikeyan is general secretary of United Workers of Petroleum Industry and was the NTUC's nominee for the Nominated MP post in the last Parliament.
Mr Lye is general secretary of the Singapore Teachers' Union.
A total of 27 union leaders are vying for the 21 seats in the central committee. Among them are NTUC secretary-general Chan Chun Sing, who is facing his first union elections since joining the NTUC this year on a part-time basis in January and being full-time since April.
According to the NTUC constitution, the 21 elected members who are elected have to re-
elect among themselves the office bearers of the committee.
In his speech, Mr Lee urged the unionists to give a strong mandate to the team of leaders.
"At the national level, we just had our election and we have voted in a new Government," he said.
"I am honoured that Singaporeans gave my team a strong mandate to take the country forward.
"Now, it is NTUC's turn to hold your elections. And you have to give your team a strong mandate to take the NTUC forward, " he added to applause from the audience.