US President Donald Trump has described as "fake news" a report that he ordered the firing of special counsel Robert Mueller last June.
Trump backed down from the decision when his own lawyer threatened to resign.
White House counsel Donald McGahn said the sacking would have a "catastrophic effect" on the presidency.
Mr Mueller is leading an inquiry into possible Trump campaign collusion with Russia to influence the US election.
Both Moscow and Mr Trump deny this.
Mr Mueller, a former FBI director, was appointed special counsel last May to look into the collusion allegations.
His appointment came after Mr Trump had fired FBI director James Comey, saying in an interview it was because of "this Russia thing".
Mr Mueller is also looking into whether that represented an obstruction of justice, reports BBC.
