Workers' Party chairman Sylvia Lim said in Parliament yesterday that after checking the Hansard, she accepted her suspicion on the timing of the goods and services tax (GST) hike "may have been wrong".
Still, she refused to apologise or withdraw the statements she had made earlier in Parliament.
Ms Lim (Aljunied GRC) said: "I will not apologise to this House because I believe that I was doing my duty, as an MP in the constitutional role that we have been elected into, to give the Government the forum to account to the people, and for that I make no apology."
Leader of the House Grace Fu found her refusal "deeply disappointing". The exchange took place at the start of yesterday's sitting, the final day of debate for Budget 2018.
Ms Lim had said last week that she suspected the Government had intended to introduce a GST hike in Budget 2018, but backed down after "test balloons" it floated got a negative response.
Yesterday, Ms Fu said Finance Minister Heng Swee Keat "had plainly said there wasn't such an intention". In refusing to apologise, "her conduct falls short of the standard of integrity and honour expected of all members", she added.
Ms Fu told Ms Lim that if she repeats her conduct, she will be referred to a panel that looks into breaches of parliamentary privilege.
"(MPs) are not entitled to make unsubstantiated allegations without taking steps to check the facts, or knowingly maintain the allegations they have been shown to have no factual basis", she said.