Police are investigating claims of a death threat against a woman who gave evidence in the Tommy Sheridan case.
The letter arrived in a handwritten envelope, Mrs McGuire said
Fiona McGuire, who lives in Peterhead, said she had received a letter making threats against her.
The letter stated: "It's your time to pay back. We will be coming to see you b***h. RIP. Keep looking at your back."
Mrs McGuire testified against former Scottish Socialist Party leader Mr Sheridan during his successful £200,000 defamation action earlier this year.
Grampian Police confirmed that a complaint was being investigated.
Mrs McGuire said the letter arrived in a handwritten envelope.
The 32-year-old mother of three gave evidence that Mr Sheridan had joined her in an orgy at an Aberdeen hotel room.
The News of the World newspaper, which had carried reports of an alleged affair, is appealing after it lost the case.
In a statement, Mr Sheridan said: "I utterly condemn any threats to Fiona McGuire or any other individual.
"My family and I have suffered many times from similar threats and I know how disturbing it can be."