Palin, 45, the Republican vice-presidential nominee last year, was supposed to serve through the end of 2010. She said she would cede control of the state to Lt Gov Sean Parnell on July 26. Speaking outside her home in Wasilla, Palin offered conflicting signals about her intentions and her motivation.

In her tone and some of her words in an often-rambling announcement, she sounded like someone who was making a permanent exit from politics after what her friends have called a rough and dispiriting year.

But her remarks, delivered in a voice that often seemed rushed and jittery, sounded at times like those of a candidate with continued national aspirations, as when she suggested she could “fight for all our children’s future from outside the Governor’s office”.

Palin said that she had decided not to seek re-election when her term expires at the end of next year and that, given that, she did not think it was fair to her constituents to continue in office.

“As I thought about this announcement that I would not seek re-election,” she said. “I thought about how much fun other Governors have as lame ducks. They maybe travel around their state, travel to other states, maybe take their overseas international trade missions... I’m not going to put Alaskans through that,” she said.

The news conference came at the end of a week in which a Vanity Fair article about Palin brought renewed focus on many of the criticisms of her as a candidate for Vice-President under John McCain and set off a new round of recriminations among McCain’s advisors about her competence.

Palin’s announcement was another unusual marker in a tumultuous year for this first-term Governor since McCain turned her into a national figure overnight by surprising his own party and naming her his running mate.

At one point, she described how her children had voted in favour of her doing this — “Four yeses and one Hell, yeah!” she said.

But at another point she invoked a military quotation, misattributing it to Gen Douglas MacArthur, in what seemed to be an effort to wave aside any suggestion that she was abandoning the fight. “He said, ‘We’re not retreating; we are advancing in another direction’,” she said. (The remark was actually said by Maj Gen Oliver Prince Smith.)

Later in the afternoon, as questions reverberated in Republican circles about what exactly she intended to do, Palin posted a notice on her Twitter site, reading: “We’ll soon attach info on decision to not seek re-election... This is in Alaska’s best interest, my family’s happy... It is good, stay tuned.”

Palin is among a number of governors who are possible contenders for the Republican presidential nomination in 2012.

Many Republican strategists say that it would be difficult for someone to run for Governor in 2010 and turn around immediately, while running a state, and run for President in 2012. Quitting midterm set off speculation about what led her to leave so abruptly.

“Good point guards don’t quit and walk off the floor if the going gets tough,” said John Weaver, a former senior strategist for McCain. “Today’s move falls further into the weirdness category; people don’t like a quitter.”

But some of her supporters argued that this could actually provide Palin an opportunity to recover from what has been a damaging year for her, and prepare herself for the 2012 race. She has been enmeshed in continuing battles with members of both parties in her Legislature.

The sheer distance of Alaska from the rest of the country complicated her ability to take care of the most basic kind of presidential preparation work.

In addition, Palin just signed a lucrative contract to write a book.

“I think she is trying to determine how she can better get to where she’d like to be,” said Speaker Mike Chenault of the Alaska Legislature, a Republican from the Kenai Peninsula.