LATEST: There is a chorus of "definitely, maybe" messages from almost all the likely contenders for the Labour leadership. Previous contender David Cunliffe held a press conference this afternoon and, like deputy leader Grant Robertson half an hour earlier, said he has yet to make up his mind.
The only contender from the last leadership election, in December 2011, to rule himself out is finance spokesman David Parker.
Shane Jones told Maori TV he is considering standing, and Andrew Little also this afternoon said he would consider putting his name forward.
EARLIER: Labour leader David Shearer has quit.
Mr Shearer announced his resignation as party leader at a 1.30pm press conference, called at just 26 minutes notice.
No successor was announced. Details of a leadership vote will be announced within 48 hours.
Mr Shearer gave no reason for his resignation, which caught media on the hop.
The Labour leader gave no reason for his resignation. He gave a brief speech, saying no individual was bigger than the goal of winning the next election. He said there was no ultimatum, and no party vote.
Mr Shearer took no questions before chief whip Chris Hipkens took over to field procedural queries.
The press conference followed a special caucus meeting.
Some Labour MPs were described as "shell-shocked" after the meeting, others as happy.
Mr Shearer will stay on as an MP.
The pending leadership contest will be the first under the so-called "primary system". If the leadership is contested, it will be a full membership ballot with the caucus getting 40%, party member 40% and affiliated unions 20%.
Fast-track political career
David Shearer has had a tumultuous time since he was appointed as Labour’s leader in 2011, replacing long-serving MP Phil Goff.
His elevation came only two years after entering Parliament in 2009 when he won the Mount Albert by-election following the departure of former Prime Minister Helen Clark.
Before entering Parliament he spent nearly 20 years working for the UN, managing the provision of aid to countries including Somalia, Rwanda, Liberia, Kosovo, Afghanistan, Lebanon and Iraq.
On one occasion he had to negotiate his wife Anuschka's release from Somali gunmen.
In 1992 he was named (together with his wife) as New Zealander of the Year by the New Zealand Herald.
The following year he was appointed as Member of the British Empire (MBE) in the British New Years Honours list.
When he was appointed to Labour’s top job, the party hoped he would be able to counter the powerful personality of Prime Minister John Key, who continues to ride high in opinion polls and approval ratings.
However, Mr Shearer has failed to show the charisma or direction required to boost Labour’s flagging popularity.
His time as Labour leader has been marred by frequent coup speculation, with several MPs including David Cunliffe and Andrew Little tipped as possible replacements.
Gaffes such as the “man ban” proposal did further damage, while he failed to gain much traction on issues such as the GSCB legislation.
He has come under criticism from grassroots Labour membership and faced ridicule from his political opponents this week when he brought a pair of dead snapper into Parliament for question time.