The Border Patrol chief has been removed from office, a day after President Trump signed an executive order to build a wall at the U.S.-Mexico border and hire 5,000 more agents, sources confirmed to Fox News Thursday.

The Associated Press first reported Thursday that Mark Morgan told senior Border Patrol agents that he was asked to leave, and said he had chosen to resign rather than fight the removal request.

Morgan was named to the post in June and took office in October. His hiring had caused concern among the rank-and-file as he was not a former agent himself.

Sources told Fox News that the latest move was to show that Customs and Border Protection is going in a new, and more aggressive, direction.

Morgan’s last day will be Jan. 31 and sources said a new chief has already been identified. Sources say he is more in line with the Trump administration on border issues.

In a statement, Kevin McAleenan, the Customs and Border Protection's acting commissioner, praised Morgan for "his unwavering dedication to our border security mission" and "lifelong career in service to the nation."

The White House released a statement saying that the post is a political appointment and therefore "all officers understand the President may choose to replace them at any time."

"No officer accepts a political appointment with the expectation that it is unlimited," the statement said.

Morgan had clashed frequently with the Border Patrol union, which backed Trump and criticized Morgan frequently. The union was infuriated when Morgan told a Senate hearing in December that he supported a comprehensive immigration overhaul – assumed to include a path to citizenship for illegal immigrants.

Morgan later clarified his statement sying he did not support “blanket amnesty” and encouraged union members to listen to his testimony.

Agency officials told The Associated Press that Morgan appeared to embrace the job. Less than a week ago, the first message on his new Twitter account read, "Chief Morgan here -- excited to use this account to share the latest news and events of the #BorderPatrol with followers."

Fox News’ Adam Housley and The Associated Press contributed to this report.