Mr Chen Show Mao, 59, was heralded as the Workers' Party's (WP) star catch when he was unveiled as a candidate in 2011.

A high-flying international corporate lawyer and a former government scholar with degrees from Harvard, Oxford and Stanford, Mr Chen bore all the hallmarks of a candidate that at the time was more typically associated with the ruling People's Action Party (PAP).

His entry into politics was hailed as a high-water mark for the opposition and a demonstration that the PAP was not the only party in town that could attract candidates of a high calibre.

In his electoral debut in 2011, Mr Chen was part of the WP's A-team led by then secretary-general Low Thia Khiang, now 63, who left the safety of his Hougang SMC stronghold to mount an ultimately successful bid for Aljunied GRC. It was the first time an opposition party had won a GRC.

Five years later, in May 2016, Mr Chen made an unprecedented move of his own, taking on Mr Low for the post of WP leader in the party's central executive committee elections. Mr Low had never faced such a challenge since he took over the party in 2001.

This came just months after the WP narrowly retained Aljunied GRC with 50.96 per cent of the votes in the 2015 General Election.

Mr Chen's attempt was unsuccessful and Mr Low fended off the challenge with 61 votes to 45. Three months later, Mr Chen resigned as party treasurer but remained a central executive committee member.

He also did not challenge his fellow Aljunied GRC MP and then assistant secretary-general Pritam Singh for the top spot in the party's next election in 2018.

Despite the lofty expectations placed on him after his high-profile entry into politics, not everyone was impressed by his performance in Parliament.

Founding Prime Minister Lee Kuan Yew publicly rebuked Mr Chen in his 2013 book One Man's View Of The World.

The late Mr Lee wrote: "Chen, however, has not turned out to be so brilliant. In Parliament, he makes good prepared speeches, with a written script, but in the follow-up, he is all over the place."

During a press conference yesterday, Mr Singh announced that Mr Chen and Mr Low, and Mr Png Eng Huat, who represented Hougang, will not be standing as candidates in the coming general election. Mr Singh emphasised that it was a collective decision reached by the party leadership and the three MPs some time ago.

Mr Png, 58, a businessman and card-carrying member of the WP since 2006, also entered politics in 2011, when he was fielded as a candidate in East Coast GRC.

His team lost by a nine-point margin to the PAP team led by then National Trades Union Congress secretary-general Lim Swee Say.

But a year later, Mr Png strode into the national spotlight when he was picked as the WP candidate to succeed WP MP Yaw Shin Leong, who left under a cloud, in the party's Hougang base during the 2012 by-election.

That year, Mr Png beat the PAP's Desmond Choo with 62.1 per cent of the vote. He defended the WP stronghold again in 2015, but won with a smaller vote share of 57.66 per cent against PAP newcomer Lee Hong Chuang.

Commenting on the WP's announcements, political observer Mustafa Izzuddin said Mr Low's retirement as an MP was expected, but Mr Chen and Mr Png stepping down at the same time came as a surprise.

Dr Mustafa, a senior international affairs analyst at management consultancy Solaris Strategies Singapore, said the trio will nonetheless continue to make their presence felt during the campaign period.