New ACT leader Don Brash has written an open "Dear John" letter to Prime Minister John Key criticising him for a lack of courage and accusing him of governing for former opposition supporters.



In it Brash says it was with a heavy heart that he quit National to seek the ACT leadership.



"I'm forced to agree with those who say you are not running the country for the benefit of all New Zealanders, but for the former Labour and Green voters who crossed over to you in 2008 for, effectively, a three-year trial."



The letter goes through the Government's policies and criticises Key's stance on the emissions trading scheme, Maori issues and the foreshore and seabed, closing the gap with Australia, spending, failing to bringing back youth minimum rates, keeping superannuation entitlements.



"Needless to say, honouring some of your commitments would have required courage. Reversing Labour's immoral election bribes would not have been easy.''



He urged Key to use his popularity to make changes to help the country.



"What's the point of cultivating such influence unless you plan to use it to help our country? And if you won't use it now, in this time of crisis, when will you?" he asked.



A spokesman for Key said he would not comment on the letter.