Media playback is unsupported on your device Media caption Chris Christie: "We've done a lot of good together"

New Jersey governor and former Republican candidate Chris Christie is endorsing frontrunner Donald Trump for president, he has announced.

Mr Christie dropped out of the 2016 presidential race after a lacklustre showing in polls and state races.

"I'm happy to be on the Trump team and I look forward to working with him," Mr Christie said.

Mr Trump gives Republicans the best chance to win the White House, he added.

He said junior senators Ted Cruz and Marco Rubio, both running for president, were "unprepared" for the job.

There was "no question" that Mr Trump will turn around Washington, Mr Christie continued, and keep Democratic candidate and former Secretary of State Hillary Clinton from winning the White House.

Analysis: Anthony Zurcher, BBC News, Houston, Texas

Chris Christie's endorsement allows Donald Trump help assure voters wary of Mr Trump's brash style that he's an acceptable candidate.

He can also serve as an attack dog, tearing into Marco Rubio the way he did during that fateful debate in New Hampshire.

The Christie endorsement could signal a new phase of Mr Trump's presidential campaign, when prominent politicians begin to make peace with the reality of a Trump nomination.

Mr Christie, and others, may see a benefit to being among the first major figures on board the Trump bandwagon. Already rumours are floating that former candidate Mike Huckabee is on the verge of joining Mr Trump's ranks as well.

This is what happens when a frontrunner draws close to victory. And just because the man approaching the finish line is Donald Trump doesn't make it any less true.

Day one of the Republican civil war

Frontrunner Donald Trump, a businessman from New York, is leading in many state polls and has already won three consecutive state contests in New Hampshire, Nevada and South Carolina, despite never having worked in politics.

He shouldered some criticism from Mr Rubio and Mr Cruz at a Republican debate on Thursday but it is not yet clear whether this has hurt his popularity.

Image copyright European photopress agency Image caption Rubio continued attacking 'con artist' Trump on Friday

"He is rewriting the playbook of American politics because he's providing strong leadership that is not dependent upon the status quo," Mr Christie said of Mr Trump.

"I will lend my support between now and November in every way that I can for Donald, to help to make this campaign an even better campaign than it's already been."

Mr Rubio, the Florida Senator, continued to assail Mr Trump the morning after the debate.

He told CBS: "A con artist is about to take over the Republican Party and the conservative movement, and we have to put a stop to it."

And at a morning rally in Dallas, he mocked the Republican front-runner about his misspelled tweets and said he had seen Mr Trump having a "meltdown" backstage at the debate.

Republican race: Who's who?

Donald Trump: Billionaire businessman and son of New York real estate tycoon Fred Trump. Now a frontrunner for the Republican nomination after attracting previously disaffected voters with controversial, populist platform

Chris Christie: Governor of New Jersey who until this month was running for the nomination himself. He has been thought of as being from a more moderate wing of the party and had called Mr Trump's proposed ban on Muslims entering the US "ridiculous"

Marco Rubio: Florida senator and son of Cuban immigrants. Seen as the Republican establishment's challenger to Mr Trump, but has struggled to effectively counter Mr Trump's forthright style

Ted Cruz: Texas senator, favoured by evangelicals and many in the conservative Tea Party faction, has also sold himself as an "anti-establishment" candidate

Meet the candidates