Former People's Action Party (PAP) MP David Ong has written to Bukit Batok residents to apologise for his abrupt resignation last month.
"I am deeply sorry for having let you down, and for not being able to serve you for the full term that you elected me for," he wrote in a letter to residents sent to their homes.
He said that he felt a personal letter of apology was necessary, even though he had apologised in his resignation letter on March 12 and subsequently through a statement to the media.
He also thanked Bukit Batok residents for their friendship and the privilege of being their MP for the last six years.
"I am grateful also to all those who stood by me during the recent difficulties, especially my wife and family," he said.
Copies of the letter, written in the four official languages, began circulating online on Wednesday.
When contacted, Mr Ong confirmed he sent it to all homes in the Bukit Batok constituency, but declined to comment when asked if he would be involved in the by-election campaigning to fill his seat.
Mr Ong, 54, stepped down over an alleged extramarital affair with a PAP member who was one of his grassroots activists. The married woman, logistics firm sales director Wendy Lim, 41, quit the PAP a few days later.
After his resignation, Prime Minister Lee Hsien Loong said a by-election would be called "in due course".
The Singapore Democratic Party declared its intention to contest the by-election almost immediately and subsequently named its party chief Chee Soon Juan, 53, as its candidate.
The PAP announced a day later that lawyer Murali Pillai, 48, who was part of the team that stood in the opposition-held Aljunied GRC last year, will defend the Bukit Batok seat for the party.
Both candidates have been walking the ground intensely since.
Bukit Batok resident Koh See Kok, 65, felt the letter came too late as it has been a month since Mr Ong quit politics. "He should have sent the letter immediately after he resigned to show his sincerity. People have already moved on," the engineering supervisor said.
Technician Francis Ng, 51, said he had wanted Mr Ong to make a public appearance to explain himself initially, but now felt it was understandable that he needed time to get his personal life in order.
"He must have had a lot to settle with his family at that point in time. Better late than never to send the letter now," he said.