SBS Transit chief executive officer Gan Juay Kiat has tendered his resignation to take responsibility for a "personal indiscretion".
Mr Gan, 61, who has been with the ComfortDelGro-owned transport group for 11 years - first as chief operating officer, executive director, then CEO since 2010 - will serve out his notice before leaving.
The Straits Times understands that the incident involves an extra-marital relationship.
SBS Transit will appoint a new CEO in due course.
Mr Gan, who is married with grown-up children, has helped place SBS Transit as an efficient multi-modal operator. Its 15-year-old North East MRT line clocked 1.54 million train-km between delays in the first nine months of the year - double what the North-South Line managed, and emerging as Singapore's most reliable rail line.
SBS Transit chairman Lim Jit Poh said: "Juay Kiat tendered his resignation to me this morning and expressed deep regret over his actions.
"He has accepted full responsibility for his lapses in conduct, and the board has accepted his resignation."
BOARD ACCEPTS RESIGNATION
Juay Kiat tendered his resignation to me this morning and expressed deep regret over his actions... He has accepted full responsibility for his lapses in conduct, and the board has accepted his resignation.
SBS TRANSIT CHAIRMAN LIM JIT POH
HIGH STANDARDS EXPECTED
In other countries, such indiscretions are tolerated... In Singapore, we expect our public or corporate leaders to be whiter than white. It is our social norm.
MR DAVID LEONG, managing director of human resource firm PeopleWorldwide Consulting.
Mr Lim added that Mr Gan "has done much for land transport in Singapore".
Professor Lim Seh Chun, an SBS Transit director, said: "I am very surprised by his resignation. I work closely with him. We are going to lose a good CEO."
Mr Gan met staff members yesterday to inform them of his resignation. However, those contacted did not wish to comment.
Mr Gan disposed of 27,300 SBS Transit shares on Dec 12, and 15,000 more on Wednesday.
Before joining the transport operator, Mr Gan held senior positions in Ascott Group, CapitaLand, Times Publishing and General Electric.
He started his career in the Singapore Armed Forces, where he held several senior command and staff appointments, rising to the rank of lieutenant-colonel.
In 2016, he was awarded the Medal of Commendation by the National Trades Union Congress for his contributions to the promotion of harmonious labour-management relations.
Mr Gan, who was a President's Scholar and an SAF (UK) Scholar, holds a Bachelor of Arts (Engineering Tripos) from the University of Cambridge in Britain.
Commenting on the case, Mr David Leong, managing director of human resource firm PeopleWorldwide Consulting, said: "In other countries, such indiscretions are tolerated... In Singapore, we expect our public or corporate leaders to be whiter than white. It is our social norm."
In a statement to the Singapore Exchange, SBS Transit said Mr Gan will leave on March 27. It added that he holds 30,000 company shares and 150,000 share options.