By Xinghui Kok

SINGAPORE Two senior lawmakers from Singapore's ruling party have resigned over their "inappropriate relationship", Prime Minister Lee Hsien Loong said on Monday, the latest high-profile scandal in a city-state otherwise known for its political stability.

Lee said the resignations of house speaker Tan Chuan-Jin and lawmaker, Cheng Li Hui, were necessary to uphold the standards of his People's Action Party (PAP).

Resignations of senior PAP members are rare in Singapore, where the party has been in power since 1959, before the country's independence in 1965.

Tan's personal conduct had "fallen short", Lee said in a statement, and he understood the speaker's desire to step away from politics and "help heal (his) family".

Lawmaker Cheng has been in parliament since 2015. Cheng could not immediately be reached for comment and her Facebook page had been taken down at the time of the announcement.

The political upheaval follows a high-level graft probe into transport minister S Iswaran and hotel tycoon Ong Beng Seng, who were arrested last week before being released on bail. They have yet to comment on the investigations.

In June, two heavyweight cabinet ministers were cleared of wrongdoing after public scrutiny of their renting state-owned bungalows at exorbitant rates.

Separately, the opposition Workers' Party (WP) said on Monday it was looking into an "inappropriate exchange" between two of its senior members after a video surfaced online that appeared to show them holding hands in a restaurant.

Both the PAP and WP have in the past sacked members for engaging in extramarital affairs.

Events like these are unusual in Singapore, which prides itself on being corruption-free and holding politicians to high moral standards.

Political scientist Chong Ja Ian at the National University of Singapore said Monday's developments were "relatively controllable issues" that would not affect Singapore's political stability.

"What it points to is that there is a need for greater transparency in both ruling and opposition parties' system," Chong said.

Speaking to local media, PM Lee said on Monday he would nominate a new house speaker by Aug 1. He added had no plans to call an immediate general election, which is due by 2025.

(Additional reporting by Chen Lin; Editing by Martin Petty, Kanupriya Kapoor)