Former Prime Minister Helen Clark has told the UN she will step down as head of the UNDP when her term ends in April.

Clark sent a letter to staff this morning to advise them of her decision not to seek another term.

"It has been an honour and privilege for me to lead UNDP for eight years," Clark wrote in the email addressed to "dear colleagues".

Clark tweeted this afternoon: "It's official. At the end of 8 years & of two full terms as @UNDP Administrator, I will be stepping down."

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She added: "It has been an honour & privilege to serve as @UNDP Administrator for 8 years & to work with remarkable staff dedicated to #development."

Labour leader Andrew Little tweeted his acknowledgment: "Thank you @HelenClarkUNDP for your eight years of service at the @UNDP. You've done New Zealand - and Labour - proud."

Prime Minister Bill English has paid tribute to Helen Clark for her work heading the UNDP after Clark's decision not to seek a third term in the role.

English said Clark had performed admirably in what was a challenging role and it had earned her international recognition.

"New Zealand has every right to be proud of Helen Clark's tenure as Administrator of the UNDP.

"I wish her all the best in her future endeavours."

English's predecessor John Key had helped Clark in her unsuccessful campaign for the job of UN Secretary General - a campaign the Government had also financially backed. Key's enthusiastic efforts to convince his fellow leaders to back Clark had resulted in then Vice-President Joe Biden joking to Key he had thought Clark was Key's sister.

Former Labour MP Judith Tizard, a close friend of Clark, said she had "no idea" what Clark's plans were or whether she had another role lined up.

"She will be thinking about she wants to do next," Tizard said.

"There are thousands of other things she can do. Though it is sad she never got the job she really wanted [UN Secretary General]."

Tizard, the former MP for Auckland Central, hinted that Clark was likely to return to New Zealand to be close to her family.

She said Clark's father George, who lives in Waihi, turns 95 this year "and she will want to be close to him".

Clark had been in New Zealand over the Christmas period to spend time with her father.

Clark was appointed in April 2009 and her role was renewed in 2013 for a second four-year term.

Clark unsuccessfully ran to be Secretary General of the UN earlier this year - but Portuguese politician Antonio Guterres was selected for the role instead.

In her time as head of the UNDP, Clark oversaw a major restructure which ruffled feathers internally but pleased some of the donor countries to the UNDP, such as the United States, which had criticised the UN for ineffective use of money.

Clark has declined an interview, but in her announcement to staff, Clark said she would support the next leader of the UNDP.

"It is my desire to see all aspects of the organisation in a strong and sustainable state when the next Administrator assumes office."

Her departure coincides with the election of US President Donald Trump who is reportedly considering executive orders to drastically cut US funding to the United Nations.

Former British foreign secretary David Milliband, who now heads the International Rescue Committee, has been tipped as a possible successor, as has French ecology minister Segolene Royal.

Helen Clark's letter reads:

"Dear colleagues,

I am writing to advise all staff that I have informed the Secretary General that I am preparing to leave my position as Administrator at the end of my second term on 19 April.

This will allow the Secretary General to appoint a new Administrator as soon as possible. I stand ready to support the transition to the new leader of the organisation. I have full confidence in our Associate Administrator, Tege Gettu, to act as Administrator if there is a gap between my departure and the arrival of the next Administrator.

This is not my final message to staff - there is much to be done between now and 19 April. There is, for example, a timetable for tabling the draft of the next Strategic Plan to enable it to be discussed at the Executive Board's Annual Meeting on 31 May. It is my desire to see all aspects of the organization in a strong and sustainable state when the next Administrator assumes office.

These are times of change across the UN system. There are post-QCPR reviews being commissioned which may impact on UNDP. While these processes are unfolding, I urge you all to continue to deliver to the high standards for which UNDP is known. Making progress on the SDGs and on supporting national development achievements must continue unabated.

It has been a privilege and an honour for me to lead UNDP for eight years. Our staff are our greatest strength, and I will miss you all. I will offer my thanks and gratitude more fully nearer to my departure."

Helen's political timeline

• 1981:

Elected to Parliament in Mt Albert

• 1984 - 90:

Minister in Labour Government, including Health and Housing, deputy Prime Minister in 1989.

• December 1993:

Rolls Mike Moore as Labour leader after National formed Government after a close-run 1993 election.

• 1996:

Low polling prompts Labour MPs including Annette King, Michael Cullen and Phil Goff to ask Clark to stand down. Clark refuses.

• 1999:

Labour wins election, Clark becomes Prime Minister.

• 2001:

Refuses to send troops to Iraq for US-led war after September 11 bombings. Clark deploys SAS troops to Afghanistan and in 2003 sends the reconstruction team to Bamiyan province, visiting them later that year.

• 2008:

National wins election, Clark stands down as Labour leader on election night.

• March 2009:

Clark is confirmed as Administrator for the UN Development Programme.

• April 2014:

Clark is renewed for a second four-year term at UNDP

• 2016:

Clark campaigns to be UN Secretary General but the role goes to Antonio Guterres.