The People's Action Party (PAP) activist who allegedly had an extramarital affair with former Bukit Batok MP David Ong has resigned from the party.

Party sources said Ms Wendy Lim, 41, left the party voluntarily. The grassroots activist in Mr Ong's single-seat constituency was among those who campaigned for him at last September's General Election.

Her resignation came three days after Mr Ong, 54, admitted to a "personal indiscretion" and resigned as MP and from the party last Saturday.

Speaker of Parliament Halimah Yacob said at the start of Monday's sitting of the House that she received Mr Ong's resignation, and declared his seat vacant. Prime Minister Lee Hsien Loong said in an earlier statement that he would call a by-election "in due course".

The alleged affair between Mr Ong and Ms Lim apparently started after the 2015 General Election. The Straits Times understands that it came to light only recently.

After news of the alleged affair broke last Saturday, Ms Lim made her Facebook page private, then later removed it completely. A short write-up about her on the website of the logistics company where she works as a sales director was also removed.

She has reportedly taken a leave of absence and her company has said she is attending to a personal matter.

Both Mr Ong and Ms Lim are married with children.

A notice in the Government Gazette yesterday announced that Mr Ong ceased to be chairman of the Jurong-Clementi Town Council from last Saturday. Jurong GRC MP Ang Wei Neng has been appointed to the role.

Senior Minister of State for Home Affairs and National Development Desmond Lee, also a Jurong GRC MP, is looking after Bukit Batok until a by-election is called.