Travel for Trump administration officials on government-owned, rented, leased, or chartered aircraft, will now require prior approval from White House chief of staff John Kelly, after it was revealed that multiple senior officials in the Trump Cabinet were using private jets and military aircraft to conduct business on the taxpayers' dime.

Mick Mulvaney, director of the Office of Management and Budget, revealed this point in a letter he sent out Friday reminding staff of administrative policies regarding travel after President Trump accepted Health and Human Services Secretary Tom Price's resignation.

"Beyond the law and formal policy, departments and agencies should recognize that we are public servants," he wrote. "Every penny we spend comes from the taxpayer. We thus owe it to the taxpayer to work as hard managing that money wisely as the taxpayer must do to earn it in the first place."

"Therefore, all travel on Government-owned, rented, leased, or chartered aircraft, except space available travel and travel to meet mission requirements (as those situations are defined in Circular A-126) shall require prior approval from the White House Chief of Staff," he added.

"Full-time required use travelers are exempted from the requirement," the letter adds.

Price's resignation came after it was revealed he used nearly $1 million in taxpayer dollars to pay for travel on private jets and military aircraft over the past few months. Price has said he will reimburse the government for his seats on these flights – totaling approximately $52,000. Other Cabinet officials are facing similar scrutiny over their use of private or military aircraft to travel instead of commercial flights, including Treasury Secretary Steven Mnuchin, EPA Administrator Scott Pruitt, and Interior Secretary Ryan Zinke.

Mulvaney noted that just because a practice was legal did not necessarily make it right.

"Accordingly, with few exceptions, the commercial air system used by millions of Americans every day is appropriate, even for very senior officials," he said.