Media playback is unsupported on your device Media caption Rajini Vaidyanathan looks at whether Sarah Palin will be an asset or liability to Donald Trump

Donald Trump's Republican presidential bid has received the backing of Sarah Palin, the populist ex-governor of Alaska who was the Republican vice-presidential candidate in 2008.

"Are you ready to stump for Trump?" Ms Palin asked cheering supporters at a campaign rally in Iowa.

She was John McCain's running mate in 2008 before they lost to Barack Obama.

Despite retiring from politics in favour of a media career, she remains an influential conservative voice.

Officially announcing her endorsement, Ms Palin said Mr Trump was someone ready to let US troops "kick Isis' ass", using another name for the self-styled Islamic State group.

"We are ready for a change," she told the rally in Ames, Iowa.

Media playback is unsupported on your device Media caption Sarah Palin asked crowds in Iowa, "Are you ready to stump for Trump?"

"He (Trump) is beholden to no one but 'we, the people'. He is perfectly positioned to let you make America great again."

The New York businessman, who leads the Republican race, said in a statement that he was "proud" to receive her backing.

Ms Palin was a "trusted conservative" with a "proven record of being fiscally modest, staunchly pro-life and [she] believes in small government that allows businesses to grow and freedom to prosper", the statement added.

Analysis - Anthony Zurcher, BBC News, Washington

While the former Alaska governor's reputation has been diminished since the heady days of 2008 - thanks to her absence from public office and involvement in series of questionable reality television shows - she is still well liked by conservative Republicans and nearly universally known.

Her backing could help insulate Mr Trump against charges that he's not a true believer in the cause due to his support for liberal political issues and candidates in the past.

At the very least, she could add even more crowd-drawing power to a Trump campaign that already fills entire sports arenas.

Read more from Anthony: Will Palin help Trump?

Mrs Palin was just two years into her Alaskan governorship when she was picked by John McCain to be his running mate.

The self-described "hockey mom" soon garnered huge crowds and massive media attention.

After the election in 2009, she resigned as Alaska governor and has since forged a lucrative career as a writer and political commentator.

Palin's speech in quotes

"How 'bout the rest of us? Right wingin', bitter clingin', proud clingers of our guns, our god, and our religions, our Constitution. Tell us that we're not red enough? Yeah, coming from the establishment. Right."

"Well, Trump, what he's been able to do, which is really ticking people off, which I'm glad about, he's going rogue left and right, man, that's why he's doing so well."

"He is from the private sector, not a politician, can I get a "Hallelujah!"

"Exactly one year from tomorrow... President Obama will be able to look up, and there, over his head, he'll be able to see that shining, towering, Trump tower. Yes, Barack, he built that, and that says a lot."

Ahead of the announcement, Mrs Palin tweeted a link to an article by her daughter Bristol attacking Mr Trump's main rival for the key Iowa caucus, Ted Cruz.

Iowa is the first chance for voters to have their say in the nomination race.

Mr Cruz has himself praised Mrs Palin saying "without her support, I wouldn't be in the Senate" - a reference to her backing that helped him to his surprise victory in a 2012 Senate run-off election.

"Regardless of what she does in 2016, I will always be a big fan," he tweeted.

Democratic frontrunner Hillary Clinton's campaign team tweeted caustically after the announcement to Sarah Palin "congrats to the Youtube commenter who wrote your remarks".