Donald Trump's daughter and senior advisor Ivanka Trump used a personal email account to send hundreds of mails to her assistants, according to US media reports.
The Washington Post, quoting anonymous sources, said the discovery was made during a review of emails by White House Officials.
Using a personal email address for government business is in violation of federal records rules.
A spokesperson for Mrs Trump's attorney insisted none of the emails contained classified information.
Donald Trump has repeatedly criticised Hillary Clinton for using a private email server, with the attacks continuing after the 2016 presidential election.
Hillary Clinton’s Emails, many of which are Classified Information, got hacked by China. Next move better be by the FBI & DOJ or, after all of their other missteps (Comey, McCabe, Strzok, Page, Ohr, FISA, Dirty Dossier etc.), their credibility will be forever gone!
— Donald J. Trump (@realDonaldTrump) August 29, 2018