Experts see move as Trump’s latest attempt to shift power of the purse away from Congress and into executive branch

By ordering that US military personnel receive paychecks even though the government is shut down, Donald Trump is seeing to the needs of a politically untouchable constituency that has been caught up in the congressional logjam over federal spending.

But experts who spoke to the Guardian warn that he is doing so in a way that is almost certainly illegal and, if left unchecked, bodes ill for Congress’s constitutional authority to control government spending. Some fear it could set the stage for the president to unilaterally fund other contentious decisions in the future, such as the deployment of the military on US soil.

“I’m with the people who believe that there’s really no good legal justification for moving the money around in this way,” said Phil Wallach, a senior fellow at the right-leaning American Enterprise Institute who focuses on the separation of powers in the US government.

“There’s no congressional authorization for troop payment for this new fiscal year. So it’s going pretty far out on a legal limb, and just sort of daring anybody to do anything about it. Because, of course, substantially, nobody thinks it’s very bad to pay the troops.”

The federal government shut down at the start of October after Democrats and Republicans in Congress failed to agree on legislation to extend funding beyond the end of September. About 700,000 federal workers have been furloughed, while hundreds of thousands of others continue to report to work, but are not getting paychecks.

Troops were paid during previous government shutdowns because Congress had either approved defense department spending, or passed bills specifically to guarantee their salaries, said Bobby Kogan, a former White House office of management and budget official who is now with the Center for American Progress, a liberal thinktank.

Congress took neither of those actions this time, though lawmakers have unsuccessfully tried to get legislation addressing federal workers’ pay through Congress, as recently as this week.

Last week, Trump acted unilaterally, repurposing a reported $8bn in defense department funds meant for research and development to pay the military in the middle of October. Kogan called the decision “super duper duper illegal” under federal law.

“If you try to spend money you have on the wrong purpose, you’re in trouble. And if you try to spend money into a purpose that you don’t have money for that purpose, then you’re in trouble as well,” he said.