Donald Trump may have committed “serious” ethical violations by giving three officials including his close personal aide Natalie Harp cash gifts of $45,000 each, according to a former White House “ethics czar”, as a watchdog group called for an investigation.

The nonpartisan Campaign Legal Center filed a complaint with the US Office of Government Ethics over the US president’s handouts to a handful of staff, arguing that an investigation is “clearly warranted”.

Norm Eisen, who was a special counsel to Barack Obama for ethics and government reform, said the payments were a possible breach of regulation 18 USC 209, a federal conflict of interest statute that prohibits the US government’s executive branch employees from receiving payment beyond their salary for performing their official duties.

The US president appeared to disregard that provision when he presented the payments to Harp, Chamberlain Harris and Margo Martin. The three aides reported the money on their financial disclosure forms as a “cash gift for holiday”.

The payments amounted to 30% of each of the aides’ $150,000 annual salary. The Washington Post first reported the story.

The White House has denied that they represent a breach of ethical codes.

But Eisen said the denials strained credibility. “It appears to raise the most serious issues under 18 USC 209, which prohibits supplementation of income,” he said. “The White House denies that these payments represent income supplementation but that is hard to believe under the circumstances.

“We’re not talking about some small gift or minor trinket, but about large sums.”

James Thurber, another ethics specialist and a founder and former director of American University’s Center for Congressional and Presidential Studies, called the payments “legal but totally unethical”, adding in an email: “This is nothing new for Trump.”

“The American public has a right to know where government employees’ loyalties lie,” said Kedric Payne, vice-president, general counsel and senior director for ethics at Campaign Legal Center. “Laws prohibiting government employees from receiving supplemental income are not arbitrary; they exist to ensure that public servants’ priorities remain focused on public service, not a wealthy benefactor who may have other objectives in mind.

“An investigation into this matter is clearly warranted, and, if a violation is indeed found, this matter must be referred to the Department of Justice.”

The statute states: “Whoever receives any salary, or any contribution to or supplementation of salary, as compensation for his services as an officer or employee of the executive branch of the United States government … from any source other than the government of the United States, except as may be contributed out of the treasury of any State, county, or municipality … shall be subject to … penalties.”

These can include fines or imprisonment, depending on whether the breach was deliberate. The burden falls on both the giver and recipients of any payment deemed illicit.

Eisen – now executive chair of the Democracy Defenders Fund, a non-profit group that has filed multiple law suit against the Trump administration – added: “There is a reason for these rules: we want our government employees first loyalty to be to the United States, and not to whoever happens to be its president. This is profoundly concerning.”

The White House did not respond to requests for a response to Eisen’s comments.

Harp, 35, who has been dubbed the “human printer” after she was said to carry a portable device to print out social media posts and articles favorable to Trump, has been the subject of recent public scrutiny because of the nature and intensity of her relationship with him.

She was revealed to have sent the president gushing emails and accompanied him when he stealthily switched planes via a catering truck after a Nato summit in Turkey on the basis of a supposed assassination threat from Iran.

Martin, a 31-year-old communications adviser, films Trump’s daily activities for social media posts. Harris, 26, is an executive assistant and was appointed last February to the US Commission of Fine Arts, which oversees Trump’s pet projects, including his controversial White House ballroom.

Additionally, Walt Nauta, the director of Oval Office operations, reported a $20,000 gift from Trump. Nauta, who earns an annual salary of $175,000, also accompanied the president on the plane switch. He served as a valet during Trump’s first presidency and was a key witness and defendant in the Department of Justice’s investigation of Trump’s handling of classified documents at his Mar-a-Lago home, led by the special prosecutor Jack Smith.