Labour leader Andrew Little believes John Key is lying about when he first knew of allegations concerning former MP Mike Sabin.
Mr Little told RadioLive this morning he did not accept the Prime Minister would not have known about the allegations before his chief of staff told the Prime Minister's office on November 26.
"I don't accept what [Mr Key]'s saying, I don't believe he's telling the truth... [he is] a liar," Mr Little said.
Mr Little told RadioLive "arrogance", and a "cavalier attitude" may have been behind Mr Key's account of the timeline of events around the allegations and Mr Sabin's resignation.
Earlier Mr Little had upped his criticism of John Key's handling of the Mike Sabin affair, saying it wasn't credible that the Prime Minister did not know about allegations concerning the National MP until late November or early December.
"In our little democracy, the Prime Minister pretty much knows everything and the Police Minister is briefed by his department on sensitive issues, so I just do not accept senior ministers of the Government did not know about this."
Mr Little landed a minor hit on Mr Key by proving his chief of staff, Matt McCarten, had told the Prime Minister's chief of staff, Wayne Eagleson, of the allegations a week earlier than Mr Key was saying he had.
Mr Sabin resigned as an MP last Friday "due to personal issues that were best dealt with outside Parliament". A by-election will be held in his Northland electorate on March 28.
One News last night reported police had been looking at the allegations since early August.
Mr Little yesterday said he first heard about Mr Sabin's personal matters soon after he became Labour leader on November 18.
It was clear from the information received by his office that the allegations had been around for some time.
But he stopped short of claiming the Prime Minister knew of the allegations before last September's election or before Mr Sabin was appointed chairman of Parliament's law and order select committee in October.
Mr Little said he decided to notify the Prime Minister's office and his chief of staff did so at the end of November.
"The issue here is that there was a serious allegation about a National MP. I considered that the leader of that party should investigate and, if necessary, take action."
Yesterday morning, the Prime Minister said Mr Little was mistaken and that Mr McCarten had contacted Mr Eagleson about the allegations on December 3.
But phone records provided to Mr Little by Parliamentary Service staff yesterday afternoon show Mr McCarten rang Wayne Eagleson on the morning of November 26.
Mr Eagleson was away from his phone. Mr McCarten left a message and Mr Eagleson rang back about 20 minutes later.
A spokeswoman for the Prime Minister said Mr Eagleson's recollection was that the conversation with Mr McCarten occurred in early December, but he was prepared to accept it was a week earlier.
Mr Key said it would have been "useful" if Mr Sabin had told the party about the personal matter earlier, "but he didn't and we didn't know about it".
November 26 Phone records show Labour leader's chief of staff contacts his counterpart in Prime Minister's office to tell him of allegations against Northland MP Mike Sabin. Prime Minister's office says chief of staff was already aware of them.
December 1 The day John Key says he first became aware of allegations against Mike Sabin.
December 3 The day Key says the chiefs of staff were in contact. Key's office later accepts this was wrong.
December 21 First media report of police investigation into allegations against Sabin.
January 30 Sabin resigns as Northland MP.
February 2 Key announces Northland by-election will be on March 28.
MS: So John Key is lying?
AL: I don't believe what he's saying, because of the information I've received.
MS: So, he's not being straight, you don't believe what he's saying, in other words he's a liar.
AL: Well that's one description for it, yes.
MS: But not a decsription you're willing to utter?
AL: I don't accept what he's saying, I don't believe what he's saying because of the info I've got and his story has shifted now because he's said he might have been told a week earlier. the issue is when he did know because he allowed Mike Sabin to take up the role of chair of law and order select committe, which he never should have.
MS: I get all of that, but I can't understand why you're reluctant to come out and say straight I believe John Key's a liar.
AL: Well I don't believe what he's saying, I don't think he's telling the truth, so, yeah I guess that's the only other conclusion you can draw is that that's exactly what he is.
MS: Is what?
AL: A liar, Mark.