In the letter sent to Mr Key yesterday, Dr Brash outlines why he has resigned from the National Party and joined ACT.
"It was with a very heavy heart that I felt obliged to resign my membership of the National Party and to seek the leadership of the ACT Party," Dr Brash said."
"I reached my decision after watching with mounting dismay the performance of your Government.
"You made great play of your ambition for New Zealand, and your determination to close the trans-Tasman wage gap and staunch the flow of our best young minds to more successful countries.
"Yet you have done almost nothing to fulfil that ambition, and now appear to have given up on that goal.
"I have not."
On Government borrowing, Dr Brash said the Government had done almost nothing to wind back "wasteful" spending by the last Labour Government.
"Two and a-half years on, the ratio of government spending to the size of the economy is higher now than it ever was under Labour.
"As a result, the Government is borrowing over $300 million a week. That's $300 added to the debt of every New Zealand family, every week.
"That is totally irresponsible. It's what Labour voters voted for, not National voters."
Dr Brash slated Mr Key for getting rid of the minimum youth wage, reversing his position on the Emissions Trading Scheme, failing to gradually raise the age of eligibility, widening rather than close the trans-Tasman wage gap and failing to abolish separate Maori electorates.
"And so John, 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.
"And, of course, for the Maori Party MPs, for whose support you seem prepared to trade away a vast treasure chest of our nation's coastal mineral wealth."
Dr Brash said Mr Key had spent the past three years "building up probably the greatest reserves of prime ministerial popularity in New Zealand history. 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?"
