UPDATE 12.16PM: An emotional Bill English has stepped down as National Party leader.

English made the announcement at a press conference at Parliament with many MPs standing behind him. HIs wife Mary and sons were also there.

He told caucus of his decision earlier this morning.

English says he had been reflecting over summer on his decision.

English says he would embark on "new personal and professional challenges."

READ MORE: Bill English's full resignation speech

He will resign and leave Parliament on February 27 and deliver a valedictory.

He says it would give National's new leader the time to prepare for 2020.

English says it had been a privilege and honour serving in Parliament for the last 27 years.

English says his proudest achievements included social investment and handling the New Zealand economy through the Global Financial Crisis.

English thanked the public for the support they had given National. He also thanked his colleagues and the National Party.

"I've been fortunate to be able to come to work every day with a sense of mission.

"To our supporters, thank you for keeping the faith through nine years of successful government and through that change of government."

READ MORE: Who could be National's next leader?

English got emotional when thanking his family, saying it was their "strength and tolerance" that had enabled his career.

English says it was not a concession of defeat to Labour leader Jacinda Ardern, and he believed National could be successful in 2020. "I'm handing over the party in as strong a position as it could be."

English will remain leader until his resignation takes effect of February 27. He would not say who his preferred successor was.

English says his decision crystallised over the summer period, a time he says was the first time in a long time when had had not faced political concerns.

He says his family had spent most their lives with the demands of politics, and he had decided he wanted to give them some time without it.

He had told his deputy Paula Bennett and finance spokesman Steven Joyce a week ago.

READ MORE: English's political career with National

English says his caucus had done him "an enormous favour" by allowing a clean handover after former Prime Minister John Key quit and he hoped the handover from him would be as tidy.

English had not let on to his caucus at the retreat last week because he had not wanted to distract from the main aim of that meeting, which was strategy and political development.

English says he would cast a vote on the leadership. "And I look forward to them coming to me to ask for it after years of having to ask for it myself."

Asked what clear moment he had decided, he joked it was "sitting on the couch at Dipton looking out at the scenery and thinking 'wouldn't it be great if I didn't have to answer media questions?'"

He says, "What matters in politics is you be satisfied yourself with what you do." He says there had been "ups and downs" but not regrets.

Although he was given Key's endorsement, he had decided it was for caucus to pick the new leader. He would pursue personal and business interests. "I"m pretty open to all those opportunities."

Asked if a return to farming was possible, he replied "I'm certainly going to spend more time back in Dipton. Whether they would think my skills were useful and relevant is another question."

English says he had told caucus that what National had over the past decade was "not normal in politics" and they should strive to maintain that stability. He says tearing itself apart was "a recipe for staying in Opposition."

English says he was not a great believer in legacies but he hoped the economy would stay strong and Labour would not dismantle the social investment approach, identifying and targeting the vulnerable in society.

He says he did not want to prejudge whether a change of leader would make it easier for National to deal with NZ First or other possible support partners in 2020.

English said when first entered Parliament Muldoon was leader and it was a "rambunctious" place and "one in which the leader of NZ First thrived." He said such instability and lack of discipline would be fatal for a party today.