It is unproductive for the Workers' Party to continue arguing with the People's Action Party on the cleaning of hawker centres, said WP chief Low Thia Khiang on Saturday.

"We have made our position known both inside and outside Parliament and the relevant documents are already in the public domain, so the public can make its own judgment on the matter," said Mr Low in a statement.

His MPs' consciences are clear, he added, and allegations against their integrity in a statement by the Prime Minister on Friday have been fully responded to and rejected.

His remarks come a day after Prime Minister Lee Hsien Loong urged the WP to clear the doubts over the integrity of its MPs.

The saga started in May when hawkers under a WP-run town council said they were asked for extra payment for the cleaning of high areas in their markets.

Such cleaning is done and paid for by town councils.

The WP denied making the demands, and the matter escalated to a dramatic exchange in Parliament on Tuesday where Environment and Water Resources Minister Vivian Balakrishnan charged that WP MPs Pritam Singh and Sylvia Lim were false and untruthful to cover up what the town council did.