He is the man the PAP sends in when it has a tough fight on its hands, in a single seat and near to the polls.

Mr Charles Chong, 67, Deputy Speaker of Parliament and the ruling party's longest-serving backbencher, is retiring after 32 years.

He won by the "narrowest of margins" in the last two elections not because he was a weak MP, but because he could be relied on to "fight hard and fight smart" in a tough contest, Prime Minister Lee Hsien Loong said yesterday in tribute.

In 2011, Mr Chong won Joo Chiat SMC with 51.02 per cent of the votes against Mr Yee Jenn Jong of the Workers' Party (WP).

In 2015, he wrested Punggol East back from the WP's Lee Li Lian with 51.76 per cent of the votes.

"Always by the skin of his teeth... because when there is a tough fight, and we need a strong candidate who will fight hard and fight smart, we send in Charles Chong," PM Lee said. "And every time, Charles has delivered."

PM Lee also paid tribute to Transport Minister Khaw Boon Wan and Emeritus Senior Minister Goh Chok Tong, whose retirements he had announced earlier.

"Boon Wan has been in all my Cabinets from day one, when I became PM; we have fought and won many battles together," he said.

PM Lee described their comradeship as "exceptional", adding that "the warmth and mutual regard we have for each other has grown over many years as we worked together, dealing with the spikiest of problems".

As for Mr Goh, PM Lee said he owed Singapore's second prime minister "a huge debt of gratitude, because he was the one who brought me into politics, and has guided and mentored me all these years, even after I succeeded him as PM".

In 44 years as MP for Marine Parade, 14 of them as prime minister, Mr Goh, 79, "has touched the lives of so many Singaporeans, and given his all in service of our country and people".

With the PAP fielding 27 new faces, about 20 MPs will be retiring this year, said PM Lee.

Each of them has served Singapore and the PAP loyally, he said, and has agreed to step down as they understand the need for self-renewal and had put the party and nation before self.

Among those likely to step down are former office holders, Mr Lim Swee Say, 65; Mr Lim Hng Kiang, 66; Professor Yaacob Ibrahim, 64; and Mr Lee Yi Shyan, 58.

Backbenchers who are expected to quit politics include Bukit Panjang MP Teo Ho Pin, 60; Jalan Besar GRC MP Lily Neo, 66; Pioneer MP Cedric Foo, 59; Mountbatten MP Lim Biow Chuan, 57; and Marine Parade GRC MP Fatimah Lateef, 54.

Speaking later, Mr Chong said his 32 years in politics have been most meaningful.

He added: "We are now in the midst of one of the most challenging times in our history.

"But I leave with the confidence that I will be handing over to capable leaders who, with the support of a resilient people, will get us through these difficult times."