The White House late Wednesday moved to terminate Robert E. Primus, a board member of the federal regulator responsible for approving railroad mergers, including one currently under review between Union Pacific and Norfolk Southern.

Mr. Primus, a Democrat who was nominated for the role by President Trump in 2020, said in an interview that his firing from the Surface Transportation Board came “completely out of the blue.” He said he arrived home from an Amtrak event after 5 p.m. on Wednesday to discover that his work phone was locked.

While reading through his personal emails, he saw one from the White House that said his position had been “terminated, effective immediately.” Mr. Primus shared a copy of the email with The New York Times. No explanation or cause was given in the email.

By Thursday, his name and biography had been removed from the S.T.B. website’s list of current board members. Mr. Primus said he would continue his work, and explore legal options if he were prevented from doing so.

“As far as I’m concerned, I haven’t received cause for the termination, so I don’t believe it to be valid,” Mr. Primus said. “It’s illegal. The president cannot remove me arbitrarily, there has to be a reason, there has to be cause.”

Kush Desai, a White House spokesman, said in statement, “Robert Primus did not align with the President’s America First agenda, and was terminated from his position by the White House.” He added, “The Administration intends to nominate new, more qualified members to the Surface Transportation Board in short order.”