The Border Patrol chief has been forced out a day after Donald Trump announced an ambitious plan to build a wall at the Mexican border and hire 5,000 Border Patrol agents, a US official told the Associated Press.

Mark Morgan told senior Border Patrol agents about his ouster during a brief video conference on Thursday morning, saying he was asked to leave and that he decided to resign rather than fight the request, according to the official, who was on the call. The official spoke on condition of anonymity because the discussion was not intended to be made public.

Customs and Border Protection said Morgan’s last official day in office would be Tuesday.

Morgan was named to the post in June and took office in October. The former FBI agent briefly led the internal affairs department at the Border Patrol’s parent agency before heading the agency of roughly 20,000 agents.

In a statement, Kevin McAleenan, US 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”.

Morgan leaves office only seven months after being named the first outsider to lead the agency since it was founded in 1924.

From the start, he clashed with the Border Patrol’s union, which endorsed Trump early and forcefully during the presidential campaign. The National Border Patrol Council had advocated for an insider to lead the agency and sharply criticized Morgan at almost every turn.

The former official said Morgan had not been at work on Wednesday and had not attended a gathering at the homeland security department with Trump and the newly appointed homeland security secretary, John Kelly.

During that visit, Trump singled out the union’s president, Brandon Judd, while pointedly avoiding mention of Morgan. Judd served on Trump’s transition team.

The union was incensed when Morgan told a Senate hearing on 1 December, in response to a question from Senator Tom Carper, a Delaware Democrat, that he supported a comprehensive immigration overhaul, which is often interpreted to include a path to citizenship for people who are in the country illegally. Morgan clarified his remarks in a note to Border Patrol staff the following week.

“I want to be clear what my position is regarding immigration reform,” Morgan wrote. “I do not, as some have suggested, support what is often referred to as ‘blanket amnesty’. This assertion could not be further from my position. I encourage everyone to listen to my testimony.”

Despite pressure from the union, many agency officials said 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.”