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Labour would not support National's plan for a referendum on MMP and Prime Minister Helen Clark has accused the party of wanting "unfettered power".
National's leader John Key yesterday promised a government led by his party would hold a binding referendum on MMP no later than 2011.
If required, a second referendum would be held to establish what system should replace it.
But given that many minor parties rely on MMP for their place in Parliament, a future minority National government could struggle to win sufficient support for a referendum.
If that was the case they would then need Labour's support.
But Miss Clark today said that was very unlikely.
"It is not in our policy to have a referendum on MMP. If it aint broke don't fix it is the usual formula and it isn't broke." "What the National Party really want is unfettered power. They don't like the thought of having to share power with other parties.
"Now I've been in the position as prime minister for close to nine years where we have shared power with other parties and by and large I've come to the conclusion that is not a bad thing." Mr Key yesterday outlined National's plans for a referendum as one of 10 election pledges in a speech to his party's annual conference.
He said there had always been the expectation when MMP was put in place that people would get to have another say further down the track.
Mr Key said because two referenda might be needed any change was unlikely until the 2014 election or "possibly later".
Mr Key said he believed voters would reject MMP.
"I think the country may well vote MMP out, but I think they will vote in another proportional system," he told reporters. "I don't think they'll go back to first past the post."