During the consultation process in New Zealand, tobacco-producing countries Nicaragua, Dominican Republic and Indonesia expressed strong opposition to the plain packaging proposal.
Australia is also facing a separate challenge from tobacco company Philip Morris, which claimed its plain packaging laws breached a free-trade treaty with Hong Kong.
Mr Key was asked yesterday about the significance of the Trans-Pacific Partnership (TPP) negotiations, which could include provisions to allow multinational companies such as tobacco firms to take direct legal action against Governments.
He said TPP talks were taking place "in isolation" and would not put a halt to the progress of the legislation.
A group of six American business groups including the US Chamber of Commerce said they were "deeply disturbed" that the Government was going ahead with the proposal.
"This bill, in effect, eliminates the right of a business to use its trademarks in everyday commerce. We respect the right of New Zealand to regulate in the public interest, but this is the wrong approach."
Labour Party trade spokesman Phil Goff told foreign vested interests to "butt out" of the debate.
"As much as apologists for tobacco might pretend this is a debate about intellectual property rights or removing barriers to trade, in fact it's about a Government's sovereign right and responsibility to promote good health and public wellbeing."
Act leader John Banks, on the other hand, said he would oppose the bill. He told the Herald: "No one dislikes smoking more than me". But he was against the state seizing property rights without compensation.
Treasury officials said an investor-state dispute could cost the Government $3 million to $6 million.
Plain packaging moves
Australia: Introduced plain packets 2012
New Zealand: Introduced legislation 2013
England, Ireland, Norway, Canada, Turkey: Considering plain packaging