Backing Boris Johnson to become the next prime minister would have been a "betrayal" of the country, Michael Gove has said.
London's former mayor "ducked" crucial decisions and failed to show the qualities needed to take over from David Cameron, according to the Justice Secretary.
Explaining his shock decision to withdraw support from Mr Johnson, a move which has been branded treacherous by some Tory colleagues, Mr Gove insisted he had been putting the country first.
"I've taken some difficult decisions, but I've always taken those because I've put my country and my principles first," he told BBC One's Andrew Marr Show.
Mr Gove said he took the decision "very late" on Wednesday evening after talking to close colleagues and wife Sarah Vine.
He insisted he had tried to ring Mr Johnson to tell him about the decision "but the clock was ticking" in the leadership contest.
"I came to the conclusion reluctantly after throwing my heart and soul for four or five days into trying to get Boris to become the next leader of the Conservative Party he could not do that job," he said.
"Right until the 11th hour I was talking to parliamentary colleagues, friends seeking to persuade them that Boris could lead the country and could be prime minister but in the final 24 hours there were actions that were taken, decisions that were ducked that led me to believe...
Mr Gove added: "Boris had the opportunity to build a team, Boris had the opportunity to lay out a particular vision in the last 24 hours and I felt that he did not step up to that challenge."
He said: "I knew that by taking that decision all sorts of people would attack me personally but I love my country, I could not recommend that Boris was prime minister, I had tried to make that work and, therefore, it would have been a genuine betrayal of principal and of this country to have allowed Boris's candidacy to go ahead with my support."
Mr Gove said it was "telling" that Mr Johnson had failed to press on with his leadership bid.
"Boris could have chosen to go on if he wished to, the fact that he didn't, I think is telling," he said. "My judgment about what is right for this country will always guide me and on that basis I came reluctantly and with a heavy heart...
"I enjoyed working with Boris during the referendum campaign, I think he has great talents and great abilities but you need something else to be prime minister, you need that grit, that executive authority, that sense of purpose, that clarity.
"I'd hoped that Boris would show that but in the end it wasn't there."
In an uncompromising interview, he was told by Andrew Marr he was viewed as the "Frank Underwood" - the Machiavellian character from the US version of House of Cards - of British politics and had "betrayed" his friends.
But Mr Gove said putting friendship first in politics did not serve the country.
The leading Leave campaigner also insisted the decision to quit the European Union would not lead to the break-up of the UK.
"I don't believe that we will need to go down that path," he said.
"There is no appetite for a second referendum."
Pressed on his past comments describing the Northern Ireland peace process as a "moral stain" and a "capitulation to violence", he insisted the negotiations "could have been handled in a different way".
"There was a problem with the Northern Ireland peace process," he added.
Mr Gove agreed to publish his tax returns before the end of the contest.