Portman has, however, denied these allegations.
"These outrageous allegations are simply false. I took the photos given to the media nearly 15 years ago and the reality behind them is nowhere close to what is being described," Portman said.
"I have been transparent and truthful about these vile claims, but I will not further engage publicly with a coordinated smear campaign," he said in a statement.
"My commitment to public service speaks for itself. I have always put duty to country first and treated others with respect. I am deeply grateful for the opportunity to have served in the Trump Administration and will seek to ensure a smooth transition when I leave the White House," he said.
Responding to questions, Sanders said his exit from the White House would not be immediate. Portman would help in the transition.
Portman is not leaving the White House under pressure and his decision in this regard is his own, she said.