Labour looks set to pull its support for new foreshore legislation, saying it does not offer a lasting solution.
That means the future of the bill could be in jeopardy, as its majority would be so slim.
Labour leader Phil Goff told the stuff website the party was having a rethink, given the bill was likely to be revisited.
"Well, that's not good enough. If it's not a lasting solution, then why are we going through the pretence, the farce of it being so."
Many iwi don't like the new Marine and Coastal Area (Takutai Moana) Bill, a result of a Government deal with the Maori Party.
But there are divisions even within the Maori Party, with maverick MP Hone Harawira strongly opposed.
The replacement legislation returns to Maori the right to seek customary title over parts of the coast through the courts or through negotiation with the Government.
The main concern of dissenting iwi is that the test for that is uninterrupted use and occupation since 1840, which they say was too high and would rule out many applications.
Labour initially said it would support the bill but without it the vote would be very close.
Former Labour MP Chris Carter has said he would follow Labour's lead.
ACT, Mr Harawira and the Greens oppose the bill, so National would have its own 58 votes, plus four from the Maori Party and one from United Future's Peter Dunne to give it 63 votes in the 122-strong house.
Maori MPs in National and the Maori Party would face considerable pressure to change their vote and if even one did that would be enough to tip the balance.
In October, Maori Party co-leader Tariana Turia would not guarantee even her own party's support for the legislation.
She said the party would make its decision about whether to support the bill further, depending on what changes were made during the select committee process and after consultation.
"...If our constituency says this is not worth our while progressing then of course we wouldn't."
The party has also said the bill could be revisited in future should it pass.
The Government has said that if the Maori Party withdrew its support, the Foreshore and Seabed Act would remain in force.
The Maori Party was formed over opposition to Labour's foreshore legislation and its repeal was a key goal, meaning the party will more than likely vote the replacement legislation all the way through.
New Zealand First leader Winston Peters yesterday appeared before the committee considering the bill. He said it would exacerbate race relations and create a constitutional rip-off for Maori not affiliated with an iwi.