In an abrupt shift, US Senator Ted Cruz has endorsed Republican presidential nominee Donald Trump, saying he is the only candidate who can stop Democrat Hillary Clinton from winning the White House race on November 8.

The Texas senator fought Mr Trump in a bitter primary battle, marked by mud-slinging and personal insults.

Mr Cruz said he would fulfil his promise to vote for the Republican nominee and that electing Hillary Clinton would be "wholly unacceptable".

His move represents a dramatic turnaround after a primary campaign during which the two candidates traded increasingly personal insults.

Dismissing Mr Cruz's early overtures, Mr Trump had attacked the Texas senator, dubbing him "Lyin' Ted" and suggesting he was not eligible to be president because he was born in Canada. Most experts say he was eligible since his mother was American.

During the heated primary battle, Mr Trump insulted Mr Cruz's wife, Heidi, for her physical appearance.

The comments underscore a dramatic U-turn for Mr Cruz, who has referred to Mr Trump as a "trainwreck" and a "pathological liar" who could not be trusted in the White House.

Mr Trump responded to the Cruz reversal by saying he was "greatly honoured" to have the endorsement of "a tough and brilliant opponent".

But some of Cruz supporters decried the senator's announcement, including his former campaign spokesman, Rick Tyler,  "It's mourning in America for conservatives. We lost our leader today."

Meanwhile, Trump campaign manager, Kellyanne Conway, who previously worked for a super PAC supporting Mr Cruz, expressed her relief on social media.