Dorrie, whose surname is being withheld for security reasons, was a lawyer in the Illinois firm representing her father when the threat, which the FBI took seriously, was received at its offices near Chicago. 

Rupert was living under a new identity in a witness-protection programme at the time. After the threat a security review was carried out for the spy’s extended family — he has been married three times and has several children. 

Rupert is giving evidence at the trial of McKevitt, the alleged leader of the dissident republican group. He posed as an American supporter to infiltrate the organisation. Under cross-examination he alluded to the threats, although there is no suggestion that they related to the McKevitt case. 

Instead they concerned the trial in London last year of Declan Rafferty, Fintan O’Farrell and Michael McDonald, three Real IRA activists who attempted to secure an arms shipment from Iraq. They pleaded guilty, making it unnecessary for Rupert to testify. During their trial, it was revealed that the three were caught in a sting inspired by Rupert, in which MI5 agents pretended to be representatives of Saddam Hussein, who allegedly called a senior Real IRA figure offering help. 

Rafferty, O’Farrell and McDonald travelled to Slovakia to meet the “Iraqis” and gave them a shopping list of 11,000lb of plastic explosives, 2,000 detonators, 200 rocket-propelled grenades, a quantity of wire-guided missiles and 500 handguns. Last year, they received sentences of 30 years each after pleading guilty. More details of the threat to Dorrie may emerge when taped interviews Rupert carried out with American journalists are made available to McKevitt’s defence team. 

The interviews were conducted mainly by Abdon Pallasch and Robert Herguth, two Chicago Sun-Times reporters, for a planned book about Rupert’s career as an undercover agent and his alleged infiltration of the Real IRA. Pallasch, a legal-affairs reporter and specialist on Ireland, said the deal had been arranged through a New York agent but that no publisher had been found so far. 

A third journalist, Newsweek’s Flynn McRoberts, has since dropped out of the project. Pallasch said the book would be objective journalism and reflect the views of dissident republicans as well as those of the spy. He also said it would reveal all aspects of Rupert’s career. 

An Illinois court ordered Pallasch, Herguth and McRoberts to hand over their interviews. 

Pallasch said he did not believe the tapes would conflict with Rupert’s evidence in court. “I haven’t been at court or seen transcripts, but from the reports I have read there are no big contradictions,” he said. 

In his judgment, the American appeal judge stressed that Rupert did not object to giving the tapes to the McKevitt defence team. Despite this, Pallasch described handing them over as “one of the toughest decisions I have ever made” and said that he had done so only after his lawyers advised him that “if we went ahead with our original plan of going to jail to protect our journalistic rights, we might prompt the court of appeal to issue a ruling which would be very bad for press freedom”. 

Pallasch said: “When you talk to the press, there should be an expectation of privacy. There should not be an expectation that it will be turned over to the government.”