A source familiar with Ms Haley's plans has confirmed to Sky News's US partner NBC News she will exit the race today - and will not be giving an endorsement.

Instead, it is understood she will encourage Mr Trump to earn the support of the Republicans and independent voters who backed her.

It means Mr Trump is heading for what is almost certainly going to be a November showdown with the man who beat him in the 2020 election: Democrat Joe Biden.

Ms Haley, a 52-year-old former South Carolina governor, had pitched herself as a solid conservative and a younger alternative to Mr Trump, who is 77.

US election latest: Nikki Haley to exit race for Republican nomination

She had also been seen as the choice for Republicans tired of Trump's ongoing legal issues - he faces four criminal cases, including over allegations over efforts to reverse the 2020 election result and his handling of national security documents.

But Ms Haley failed to reach Republican voters, losing almost all the Republican nominating contests and securing just one of the primaries on Super Tuesday.

Mr Trump and Mr Biden each won California, Texas, Alabama, Colorado, Maine, Oklahoma, Virginia, North Carolina, Tennessee, Arkansas, Minnesota and Massachusetts.

Mr Biden also won the Democratic primaries in Utah, Vermont and Iowa. The only contest the 81-year-old lost on Tuesday was the Democratic caucus in American Samoa, a tiny US territory in the South Pacific Ocean.

Read more: We're on course for the oldest presidential race in US history - and one most people don't want

Ms Haley's only win of the night came in Vermont - denying Mr Trump a full sweep.

But the former president carried other states that might have been favourable to her such as Virginia, Massachusetts and Maine - which have large swathes of moderate voters like those who have backed her in previous primaries - and forced his last remaining challenger out of the race.

He declared the GOP was united behind him.

But Ms Haley's spokesperson Olivia Perez-Cubas said: "Unity is not achieved by simply claiming, 'We're united'.

"In state after state, there remains a large block of Republican primary voters who are expressing deep concerns about Donald Trump. That is not the unity our party needs for success.

"Addressing those voters' concerns will make the Republican Party and America better."

Ms Haley was ambassador to the United Nations for almost two years under Mr Trump's presidency and when she resigned, he called her a "very special" person.

In recent months on the campaign trail, however, he changed his tune.

He used her background as a daughter of Indian immigrants as ammunition for racially-charged attacks, and he amplified false claims about her eligibility for the White House, despite her having been born in South Carolina.

She fought back, describing him as "unhinged", saying he is too chaotic and divisive to be an effective president, and criticising his admiration for dictators such as Russian President Vladimir Putin.

Read more from Sky News:
Haley becomes first woman to win a Republican primary
How Trump's grip on the Republican Party is tightening

Her departure - after enduring a long string of losses, including her home state - clears Mr Trump to focus solely on his likely rematch in November with Mr Biden.

They and their politics will dominate the next eight months, having dominated the last eight years.

However, it is a rematch few Americans want.

Opinion polls show both Mr Biden and Mr Trump have low approval ratings among voters.