Donald Trump has appointed the US ambassador to Germany, a combative loyalist, to his administration’s most senior intelligence post, in his continuing effort to wield personal control over the spy agencies, according to multiple US reports.

By making Richard Grenell acting director of national intelligence (DNI), rather than nominating him for the permanent position, Trump has sidestepped the need for Senate confirmation, a loophole the president has increasingly exploited as he has moved to replace career officials with those chosen for their personal loyalty.

The move marks a radical break from past practice. Since the position was established in the wake of the 9/11 attacks to coordinate the 17 intelligence agencies, the office of the director of national intelligence has been viewed as non-partisan, and generally occupied by career professionals. The current acting DNI, Joseph Maguire, is a retired vice-admiral and former head of the National Counterterrorism Center.

Grenell does not have a background in intelligence or the armed services, but the White House statement confirming the appointment claimed Grenell had “years of experience” working with the intelligence community in other jobs, as special envoy to Serbia and Kosovo peace talks (a job he was given in October) and while he was spokesman at the US mission to the UN from 2001 to 2008.

“He is committed to a non-political, non-partisan approach as head of the intelligence community, on which our safety and security depend,” the statement said.

Until now Grenell has been best known as a Twitter warrior, lashing out at critics of the Trump administration with a ferocity that captured the president’s attention.

Grenell has also been an outspoken advocate of LGBT rights, and has made the issue part of his brief as ambassador.

According to some reports, he will remain ambassador to Berlin and special envoy for Serbia and Kosovo peace negotiations while overseeing the US intelligence agencies. Neither the state department nor the White House would comment on those reports on Thursday.

The president has been a bitter critic of the intelligence agencies, particularly when their assessments were at odds with his own – about Iran and North Korea, for example. He once derided agency chiefs as “passive and naive”. His denunciations became so acerbic that the agency chiefs have stopped giving public briefings to Congress over national security threats.

“The president has selected an individual without any intelligence experience to serve as the leader of the nation’s intelligence community in an acting capacity,” Mark Warner, the top Democrat on the Senate intelligence committee, said, noting that Grenell was the second acting director in the post since the resignation of the last Senate-confirmed DNI, Dan Coats, last summer.

Warner said that the acting appointments were an apparent “effort to sidestep the Senate’s constitutional authority to advise and consent on such critical national security positions, and flouting the clear intent of Congress when it established the office of the director of national intelligence in 2004”.

“This should frighten you,” the former National Security Agency lawyer Susan Hennessey said on Twitter. “Not just brazen politicization of intelligence, but also someone who is utterly incompetent in an important security role. The guardrails are gone.”

After Coats’s resignation in July, Trump attempted to replace him with an outspoken Republican partisan, the congressman John Ratcliffe, but Ratcliffe was forced to stand down in the face of bipartisan scepticism over his qualifications in the Senate and revelations that he had exaggerated his experience in his official biography.