The Workers' Party's move to transfer the vacant Non-Constituency MP seat in Parliament from one party member to another is a "political manoeuvre", said three People's Action Party MPs.
In criticising it, they said the action complied with the letter but not the spirit of the scheme, which offers a seat in Parliament to the best-performing opposition candidates who lost in a general election.
They also zeroed in on the "contradiction" between how the WP opposed the scheme while allowing its members to take up the NCMP seats offered to them.
Said Minister in the Prime Minister's Office Chan Chun Sing: "We want to see the third NCMP seat filled. What troubles us is the manoeuvring behind it."
He questioned the WP's motivations in supporting the decision of Ms Lee Li Lian, its losing candidate for Punggol East, to reject the seat.
The NCMP scheme should not be turned into a revolving door to showcase talent for a party's own objectives, argued Mr Chan, the labour chief and Government Whip, who ensures PAP MPs vote according to the party's line.
He criticised Ms Lee's comments made last year when she said her seat should go to a colleague as "there are better people in the party that we should showcase".
"Let us not turn the NCMP scheme from one of service to the nation into a revolving door for partisan political talent displays."
Mr Chan, Mr Charles Chong (Punggol East) and Ms Lee Bee Wah (Nee Soon GRC) also charged the opposition party with trying to have its cake and eat it.
They took aim at the contradiction between the WP's opposition to the scheme and its candidates continuing to take up NCMP seats.
Mr Chan pointed out there are political merits for any political party to take up the NCMP scheme, which has benefited the WP.
Otherwise, the WP's members would not have taken up the seats, he added, noting Ms Sylvia Lim did so in 2006 and Mr Yee Jenn Jong and Mr Gerald Giam, in 2011.
Mr Chan said: "We have to recognise that the WP has continued to criticise the system and yet deliberately made use of it... for its political advantage."
Mr Chong cited WP chief Low Thia Khiang's declaration that he would resign if he were made to take up an NCMP seat and said it sent "mixed signals from the leadership of the WP".
The MPs also referred to Mr Low comparing an NCMP to "duckweed on the water of the pond", without roots in a constituency.
Said Mr Chan: "He even called NCMPs duckweeds. I don't know how Mr Leon Perera or Mr Dennis Tan feels. But I will guarantee you that none of my PAP MPs will ever call you duckweeds. We value you for your contribution."
The question of the duty an NCMP owed to voters in the constituency he or she had contested was also raised during the debate.
The PAP MPs argued it would not be fair to the Punggol East constituents who had voted for Ms Lee to transfer the NCMP seat to the WP's candidate in East Coast GRC, National University of Singapore sociologist Daniel Goh.
Said Ms Lee Bee Wah, who spoke in Mandarin: "The WP has moved to allow the voice that's entitled by the Punggol East voters in Parliament to translate into the East Coast GRC. Will this be fair to the Punggol East residents?"
She and Mr Chong noted that such a change adhered to the letter of the law, but questioned if it kept to the spirit of the scheme.
"While the Parliamentary Elections Act allows for opposition parties to nominate the NCMPs where a GRC is concerned, the Act is not intended to let opposition parties pick and choose which best losers from different constituencies were to enter Parliament," said Mr Chong, who defeated Ms Lee Li Lian.
He said a sufficient number of Punggol East voters had given Ms Lee the mandate to enter Parliament on an NCMP seat, but Dr Goh did not qualify on the basis of the 2015 General Election results.
Said Mr Chong: "I would urge them to play by its rules, to abide by its spirit and not to use non-acceptance of a seat by properly elected candidates as a backdoor way to have someone who did not qualify for the position into this House."
He called the WP's move "a rather unfortunate misuse of the guarantee in the Constitution that at least nine opposition MPs will be offered places in Parliament".
Agreeing, Mr Chan said: "The right and the privilege was given to (Ms Lee) by Punggol East residents. It was not for her to give it to someone else."
The PAP MPs also called for the rules governing the transfer of NCMP seats to be reviewed.
Ms Lee Bee Wah argued against letting it be transferable, especially with the newly announced proposal to give an NCMP the same powers as an elected MP.
"All candidates must not have this attitude that they must be elected, and only then can they serve the electorate," she said.