Prime Minister John Key has admitted that enforcing plain packaging on cigarette packets was not a "slam-dunk" policy due to the risk of legal challenges from tobacco giants and tobacco-producing countries.

The Government is seeking consultation on proposed changes to cigarette packaging, which it hopes will discourage smoking by cutting off the tobacco industry's last avenue of marketing.

Government documents released yesterday said legal challenges from tobacco companies could cost up to $6 million per case to contest, and challenges from countries for breaches of trade could cost up to $2 million.

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