Retired neurosurgeon Ben Carson is unlikely to join the new Trump administration, although he will remain an informal adviser, the Washington Post reported.

Carson is one of Trump's primary challengers who built a close relationship with the real estate billionaire.

He was considered among the top potential picks if Trump wanted to fill out his Cabinet and circle of close advisers with more diverse faces, including minorities and women, after a campaign that fueled racial and gender tensions.

Among the roles Carson could have fulfilled is secretary of Health and Human Services. Though, in an interview, Carson told the Washington Post that he prefers to play a role outside the administration. He said he made his decision clear to Trump in several private conversations in recent days, the Post said.