Prime Minister John Key is fending off criticism that he has no mandate for the trial of controversial charter schools - a proposal neither National or Act campaigned on before the election.

The Labour Party and teacher and principal unions have rounded on Mr Key and Act MP John Banks for agreeing to the trial.

The education sector is also concerned it will be shut out of the consultation phase; the implementation group will consist of private sector, community, iwi and Government officials, supported by the Ministry of Education.

NZEI president Ian Leckie called for parliamentary scrutiny of the trial.

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"If the Government and Act are so wedded to the idea of charter schools, perhaps they should trial them in Epsom," he said, referring to Mr Banks' electorate.

Mr Key defended the trial, saying charter schools were a potential answer to schools that were failing poorer communities.

"It's a step towards more choice. The vast bulk of schools will continue to be state schools," he told Radio NZ.

He said the proposal came from Act.

"Right through the campaign we've always talked about choice in education. At the end of the day, that's MMP for you, that you agree to different proposals."

Mr Banks would not comment on why charter schools were not a part of Act's pre-election manifesto.