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Flynn's refusal to cooperate comes as he faces scrutiny in several inquiries, including on Capitol Hill and a federal grand jury that has issued subpoenas to associates of the ex-national security adviser.

Flynn's refusal to cooperate will also intensify scrutiny over Trump's decision to hire him initially for the job and his decision to keep him on staff for 18 days after the President was warned by former acting Attorney General Sally Yates that Flynn may have been compromised by the Russians.

The Senate committee had asked Flynn earlier this month to produce all records over his communications with Russian officials by this Wednesday. But Flynn is expected to send a letter later Monday invoking his Fifth Amendment rights.

The source close to Flynn said it would be "highly imprudent for him not to exercise his Fifth Amendment rights" given that several members of Congress have called for his prosecution.

Flynn's decision to decline the subpoena does not come as a surprise to Senate intelligence leaders, as Flynn's lawyer, Robert Kelner, also told the panel last month he would not provide documents in response to an April request.

During the 2016 campaign, Trump blasted aides to Hillary Clinton for taking the Fifth Amendment in relation to the investigation of her use of a private email server while secretary of state. He said at a September Iowa rally: "So there are five people taking the Fifth Amendment, like you see on the mob, right? You see the mob takes the Fifth. If you're innocent, why are you taking the Fifth Amendment?"

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Michael Flynn expected to invoke Fifth Amendment ...