WASHINGTON—A $43,000 soundproof phone booth for Environmental Protection Agency chief Scott Pruitt violated the law, a government oversight body said Monday, because the agency didn’t notify Congress before making the purchase.

The finding, by the Government Accountability Office, comes as Mr. Pruitt has come under fire for his spending on items such as first-class air travel at taxpayer expense. The White House is reviewing Mr. Pruitt’s activities, and the Office of Government Ethics has urged the EPA to investigate possible ethics violations, including allegations that Mr. Pruitt rented a Washington apartment from the family of an energy lobbyist at below-market rates.

The White House chief of staff has reportedly pressed for Mr. Pruitt’s removal, but President Donald Trump has continued to support the EPA chief, saying he is doing a “great job.”

On the phone-booth purchase, the EPA is addressing the GAO’s concern “and will be sending Congress the necessary information this week,” said Liz Bowman, an EPA spokeswoman in a statement. The agency has said Mr. Pruitt’s living arrangement in Washington didn’t run afoul of ethics rules, and it has said his first-class travel is necessary for security purposes.

In a statement on the phone booth, the GAO said agencies are supposed to notify congressional appropriations committees before they spend more than $5,000 for the office of a presidential appointee. That also means the EPA violated the Antideficiency Act, a federal law that prohibits federal agencies from spending more than is “legally available” for such items, the GAO said.