Former New York City Mayor Rudy Giuliani on Sunday said the basis for special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation is not legitimate.

"I'm not saying Mueller is illegitimate, I'm saying the basis on which he was appointed was illegitimate," he said on CNN's "State of the Union."

Giuliani, now a member of President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's legal team, said the Mueller probe was illegitimate because of the use of an informant who had discussions with Trump campaign aides in 2016.

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"We're more convinced, as we see it, that this is a rigged investigation," he said. "Now we have this whole new 'spygate' thing thrown on top of already legitimate questions."

Trump and his allies have repeatedly described the informant as a spy, sometimes saying it was someone embedded in the Trump campaign. News reports indicate the informant was an American professor in England who was not embedded within Trump's campaign.

The use of the informant happened under the Obama Justice Department and before Mueller's appointment.

Former Obama administration officials have said it was not unusual for the FBI to use informants in counterintelligence investigations.