Workers' Party chief Low Thia Khiang cannot leave grave doubts about the integrity of his fellow MPs unresolved, Prime Minister Lee Hsien Loong said on Friday, in his first comments since a dramatic exchange between one of his ministers and the opposition party in Parliament earlier this week.

"This is not how members of a First World Parliament should conduct themselves," he added, in an oblique reference to the WP's slogan in the last general election.

"Neither is this the sort of politics Singapore needs," Mr Lee said in a statement.

On Tuesday, Environment and Water Resources Minister Vivian Balakrishnan produced a dossier containing evidence that a staff of the WP-run town council in Aljunied GRC, had asked hawkers to pay for the cleaning of their market's high areas. Such cleaning is usually done and paid for by town councils.

WP chairman Sylvia Lim, who also chairs the town council, denied the charges.

Dr Balakrishnan accused the WP of being "false and untruthful", saying the crux of the matter was about integrity and clean politics.

On Tuesday, Mr Low promised to try to find out who had actually asked the contractor to quote for the cleaning. But on Wednesday, in an interview with reporters, he "reversed course without explanation", said PM Lee, when he said that he would not conduct any investigation.

Mr Low had then questioned if the exchange in Parliament was "good politics", brining up PM Lee's comment a week ago that the country needs to get its politics right.

Yesterday, PM Lee turned this argument back on the WP.

"Good politics is first and foremost about integrity," he said.

"Singapore has succeeded because we have honest, upright people in politics: people who can be trusted to uphold the public interest, to speak the truth even when it is inconvenient, and to admit mistakes when things go wrong."

This, he said, is how the Government has built trust between Singaporeans and their leaders, worked together to build the nation and improved everyone's lives.

"If we cannot trust a politician to tell the truth, then we cannot trust him or her to safeguard public funds, to put public interest ahead of personal gain, or to make decisions affecting the well-being and security of Singaporeans," PM Lee added.

Dr Balakrishnan's statement in Parliament was not just his personal opinion, the Prime Minister said, but the government's official position which the Cabinet had approved.