Progressive Party leader Jim Anderton says Peter Dunne must step down as Revenue Minister, now he has declared his support for National.
Mr Dunne, who leads the United Future Party, is a minister outside Cabinet under a confidence and supply agreement reached with Labour after the last election.
United Future has been a confidence and supply partner with Labour for the past two terms.
Mr Dunne says he will not leave his portfolio until that agreement expires.
But Mr Anderton says the arrangement is null and void now Mr Dunne has announced his intention to support another party, and he should give up his ministerial role immediately.
Mr Dunne say there is nothing wrong with him staying on as a Minister, and giving up his ministerial role would send the wrong message.
"I think the signal that would send was that we couldn't be relied upon to carry out our word," he says.
"There's more to actually being part of a government and working constructively than simply the question of whether your vote is needed, and we wouldn't be talking now about a relationship with National if it was just simply around what votes are needed. It's about a direction."
Mr Dunne says the party's policy goals are more closely aligned with National, especially when the Greens are likely to be part of Labour's line up.
National's leader John Key says no policy deals have been done, but has indicated Mr Dunne would be a minister in a National government.
Mr Dunne and National Party leader John Key made the announcement at Parliament on Sunday, 26 October.
Mr Dunne says he feels his party could better advance its policies by working with National.
"We've come to the conclusion that the best step for New Zealand, and for United Future in terms of the policies we want to see advanced, is to support a National-led government after the election, and not to support a Labour-led government."
Mr Dunne says there is a lot of presssure on smaller parties to declare their hand ahead of the election.
National now has two potential coalition partners, ACT and United Future, both of which currently have two MPs in Parliament.
Mr Key says no promises have been made in terms of policy deals but he looks forward to Mr Dunne serving as a minister in a National-led Government.
"I've always argued that if National's in a position to put together the Government it will set to reach out and work with other parties. United Future and Peter Dunne will be one of those parties that we would be working with. Any final arrangements would be subject to negotiations and discussions as I've always said after the election."
Mr Dunne says he did not contact Labour leader Helen Clark before the announcement.
Miss Clark says it is reminiscent of the last election, where Mr Dunne cosied up to National's Don Brash, but ended up supporting Labour.