Veteran Senator John McCain, a fierce critic and bitter enemy of the President-elect, admitted he had no idea if the contents of the explosive document were true.
But he said he was so concerned about the information contained within the 35-page dossier, which included allegations that Mr Trump had hired prostitutes in Moscow, that he passed the information onto the FBI.
He told CNN: “I did what any citizen should do. I received sensitive information and handed it to the FBI.
“I don’t know if it is credible or not but the information I thought deserved to be delivered to the FBI, the appropriate agency of government.
“It doesn’t trouble me because I don’t know if it is accurate or not. I have no way of corroborating that.
“The individual gave me the information. I looked at it. After receiving that information I took it to the FBI.”
Mr McCain, who ran for the White House as the Republican candidate in 2008, said he had since found out the FBI had already been made aware of the information in the dossier.
He said: “Late last year, I received sensitive information that has since been made public.
“Upon examination of the contents, and unable to make a judgment about their accuracy, I delivered the information to the Director of the FBI.
“That has been the extent of my contact with the FBI or any other government agency regarding this issue.”
The Kremlin has denied all of the allegations and Mr Trump tweeted: “Fake news. A total political witch-hunt.”
There is no love lost between Mr McCain and Mr Trump and the animosity between them is well-documented.
The pair clashed last July when Mr Trump called Mr McCain’s Vietnam war hero status into question claiming: “He’s a war hero because he was captured? I like people who weren’t captured.”
But Mr McCain did not formally withdraw his support for the billionaire New Yorke until the closing weeks of the election campaign when recordings of of apparently boasting about his sexual conquests were published.