WASHINGTON (Reuters) -Lina Khan, who headed the U.S. Federal Trade Commission under former President Joe Biden until Monday, will resign from the commission in the coming weeks, she told staff in a memo.

Khan was an aggressive enforcer of antitrust law, challenging numerous mergers and working to ensure consumers and workers were not disadvantaged by powerful corporations.

Under Biden, Khan's FTC sued Amazon, opened an investigation into Microsoft, and won court rulings that blocked Kroger's $25-billion acquisition of rival grocery chain Albertsons and the $8.5 billion merger of handbag makers Tapestry and Capri.

The youngest person to lead the U.S. consumer protection and antitrust agency, Khan gained attention in 2017 when she wrote a paper arguing that Amazon had amassed monopoly power by undercutting competitor prices and harvesting consumer data.

Republican Commissioner Andrew Ferguson is now the agency's chair after President Donald Trump took office. A source told Reuters on Monday that Khan plans to use her remaining time as a commissioner to complete document retention and records management as required by law, as well as other administrative tasks.