Mana leader Hone Harawira has accused Maori Party MPs of being the Prime Minister's "little house niggers" over their indecision about attending a water rights hui.
The Maori King, Tuheitia, has called the meeting for iwi leaders, the Maori Council and politicians to try to secure a collaborative approach on Maori rights and interests in water.
Mr Key has banned his Maori MPs from attending the hui.
Maori Party leader Pita Sharples yesterday said his party's attendance was "still to be determined".
Co-leader Tariana Turia said: "At this point I don't see the point of us going."
Mr Harawira took to his Facebook page to lash out at their reluctance to attend.
He said Mr Key was "the real leader" of the Maori Party and accused his former colleagues of falling in line behind National Party policy.
"Time John Key realised a few home truths like (1) he can tell his little house niggers what to do, but (2) the rest of us don't give a shit for him or his opinions."
Mr Harawira said the Maori Party had to consult Mr Key before making its decisions.
However, he would welcome Dr Sharples or Mrs Turia at the hui if they decided to attend.
"If they come I'll still be happy to see them - they need to see how p***ed off everyone is with these asset sales."
Maori Party MP Te Ururoa Flavell called Mr Harawira's choice of words a "Honeism".
"We don't pay much attention to Hone's comments because most of the time it is untruthful... if we worried about what Hone Harawira said at every turn we'd feel quite annoyed all the time. We're just representing our people the best we can."
Mr Flavell said Mr Key had no say into whether or not Maori Party MPs would attend the hui.
"I can give an absolute assurance that he wasn't consulted. He doesn't run the Maori Party just as much as we don't run the National Party."
Mr Flavell said his party had initiated past hui over Maori water rights and had consistently opposed the Government's planned partial asset sale programme.
Mr Key said he would be out of the country next Thursday and would instruct his Maori MPs not to go even if invited.
"If you are an MP in the Government, you represent the Crown and any representation by my MPs at such an event would be interpreted as representation by the Crown and I've made that position absolutely crystal clear: I do not accept the view that there needs to be national hui because I do not accept there will be a national settlement because I do not accept it's a national issue."
A spokesman for the King, Tuku Morgan, said invitations for Mr Key and his MPs were never considered.
"This is a national summit on water for our people to ... have a constructive discussion in our time and in our own space."
The meeting would allow Maori Council co-chairman Sir Eddie Durie to spell out the case to the tribunal "because many of our people have not heard it other than what they've read in the paper".
But several Maori politicians will attend, including from the Mana Party and Labour's Hauraki Waikato MP, Nanaia Mahuta, the King's cousin.
The Herald understands Green Party MPs will attend if invited.
Meanwhile, Mr Key said it was "extremely important that people don't get confused" about the talks his Government was to have with affected iwi about the Mighty River sale over a five-week period. The purpose of that consultation was "to confirm the point that the sale of shares in those companies in no way affects the rights and interests of Maori in water".
Maori Council deputy chairwoman Rahui Katene said Mr Key's comment called into question the Government's good faith. "If he's going into any legal action saying he showed good faith by having consultation it's going to be very easy to show that no, in fact this was not consultation."
Maori Council lawyers were primed to file a legal challenge to the Government's Mighty River sale plan, she said, but were unlikely to do so until after a September 15 meeting.