Former U.S. Republican presidential contender Ben Carson, who dropped his bid last week, plans to endorse front-runner Donald Trump on Friday morning, the Washington Post reported on Thursday, citing two people familiar with his thinking.

One of the people says Carson was torn between Trump and Texas Senator Ted Cruz, but opted to support Trump because of a rumour circulated ahead of the Iowa caucuses by the Cruz campaign that Carson had dropped out of the race.

In an interview with Fox News Radio's John Gibson Show Thursday, Carson was asked whether he was planning to join the Trump campaign.

"Let's put it this way, I'm certainly leaning," he said, adding that there are "two Donald Trumps" — the one seen on television and the one he's gotten to know behind the scenes.

Carson's longtime friend and business manager, Armstrong Williams, says Carson's backing of Trump "shows respect to the voters. Donald Trump has earned these voters."

"We need to rally behind the nominee, and you don't want to do it too late," he said. "This country's in trouble. The Republican Party needs to unify."

Trump is expected to formally announce the endorsement at a news conference Friday morning at his Mar-A-Lago Club in West Palm Beach, Fla.

The retired neurosurgeon agreed to the endorsement at a meeting with Trump at Mar-a-Lago, the people said on condition of anonymity, according to the newspaper.

With files from Reuters