Ivanka Trump, the US president's daughter and a top White House adviser, sent hundreds of emails about government business from a personal email account last year, The Washington Post reports.
The emails were sent to White House aides, cabinet members and Ivanka Trump's assistants, many in violation of public records rules, the paper said.
President Donald Trump mercilessly criticised his 2016 Democratic rival, Hillary Clinton, for using a private email server during her time as secretary of state, labelling her "Crooked Hillary" and saying she belonged in jail.
The White House did not immediately respond to questions about the email use, but a spokesman for Ivanka Trump's lawyer, Abbe Lowell, did not dispute the report.
"While transitioning into government, Ms Trump sometimes used her private account, almost always for logistics and scheduling concerning her family," said the spokesman, Peter Mirijanian.
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Mr Mirijanian stressed that no classified information was transmitted in the messages, that no emails were deleted and that the emails have since been "retained" in conformity with records laws.
"When concerns were raised in the press 14 months ago, Ms Trump reviewed and verified her email use with White House Counsel and explained the issue to congressional leaders," he said.
The discovery was prompted by public records requests from the liberal watchdog group American Oversight.
The group's executive director, Austin Evers, said in a statement that "The president's family is not above the law", and he called on Congress to investigate.
"For more than two years, President Trump and senior leaders in Congress have made it very clear that they view the use of personal email servers for government business to be a serious offence that demands investigation and even prosecution, and we expect the same standard will be applied in this case," he said.
The emails the group uncovered include correspondence between Ivanka Trump and Small Business Administrator Linda McMahon and Education Secretary Betsy DeVos.
