Donald Trump's lawyer, Rudy Giuliani, said the president did not know beforehand about a 2016 meeting between his campaign and Russians, as the president's former attorney is reportedly ready to tell prosecutors.

Giuliani said those who were allegedly present when Trump learned about the meeting also deny he knew about it. The meeting has become a central focus of special counsel Robert Mueller's investigation into whether anyone within Trump's campaign colluded with Russia to interfere in meddling in the last U.S. presidential election.

"It's just flat-out untrue," Giuliani, the former mayor of New York City, said on "Fox News Sunday" about the claims.