"Through all our time together as a family, we have lived with the demands of public service. Your strength and tolerance has enabled my career. I now look forward to our new life together."
He said he made his decision over the summer break and informed deputy leader Paula Bennett and senior MP Steven Joyce about a week ago, before telling the caucus this morning.
"I was probably sitting on the couch looking out the window in Dipton at the peaceful environment , and thinking, 'Wouldn't it be great if I didn't have to answer media questions?'
"It really just crystallised over the summer period. I spent quite a lot of time - the first time in a long time - with no real political concerns. No need to get ready for the next Cabinet. Time together with family."
He said his resignation was not a concession of defeat to Labour leader Jacinda Ardern, and he believed National could be successful in 2020.
He told caucus this morning that the stability that National had enjoyed over the past decade was "not normal in politics", and tearing itself apart was "a recipe for staying in Opposition".
"I'm handing over the party in as strong a position as it could be."
Speculation over English's leadership mounted two weeks ago when he delivered his State of the Nation speech, but he batted it away internal party rumblings as "gossip", and said his performance would determine whether he lead the party into the 2020 election.
Following today's announcement, Paula Bennett paid tribute to English and said the team would "miss him a lot", but did not say whether she would run for the leadership.
"I just don't think New Zealand will ever fully appreciate the depth of his thinking, and everything he's given to them."
Of the likely leadership contenders, only Judith Collins managed to fit into the room for English's announcement. Simon Bridges was in Tauranga because of travel delays.
English did not want to endorse a successor, nor did he say if the new leader should be an experienced MP, someone who more represents generational change, or someone who would appeal to NZ First for potential coalition deals.
"I'll just have a vote like everyone else in the process. I expect I might enjoy them coming to ask me for it, after years of having to go and ask other people for their votes."
He said serving as Prime Minister was his highlight, along with managing the economy through the global financial crisis, overseeing reforms as Health Minister in the 1990s, being Opposition education spokesman in the mid-2000s, and helping with the rebuild of Christchurch following the 2011 earthquake.
"In recent years I enjoyed the development of Social Investment and new ways of the Crown working with Māori to make a real difference.
"I'm pleased that pretty much everyday I came into this building, I had a sense about achieving things, and got on with it."
He said a lot had changed over his 27 years in Parliament.
"If I think of the National Party caucus that I first joined in 1990, which included Sir Rob Muldoon, and 36 new MPs of which there will only be one left in two weeks, and that will be Nick Smith, it was a much more unpredictable and rambunctious environment, one in which the current leader of NZ First thrived.
"That would be fatal to a Government today. People now expect cohesion and unity."
Asked by media about whether he'd support any of his children going into politics, he quipped: "I'd say it'd be much more powerful to be a journalist."
And on the chances of going into farming? "I'm certainly going to spend time in Dipton. Whether they regard my skills as useful or relevant is another question altogether."
He wouldn't rule out taking on an international role.
"I want an opportunity to be able to start again in a different life, and for our family to be able to live without politics."
