An assurance of "secrecy forever" lies at the heart of the relationship between the security service and its agents, the High Court has heard.
The assertion was made as the Home Secretary applied for her lawyers to be allowed to give evidence in secret to defend a damages claim being brought by IRA mole Martin McGartland.
The former RUC Special Branch agent is suing MI5 for breach of contract and negligence in his aftercare following an IRA shooting that left him unable to work. He claims MI5 failed to provide care for post-traumatic stress disorder and access to disability benefits.
Home Secretary Theresa May is asking Mr Justice Mitting at London's High Court to allow closed material proceedings to be used when his case comes on for hearing to protect national security. If the judge allows the application, Mr McGartland (42) and his lawyers will not be able to hear parts of the case or to see "sensitive material".
They are also challenging the Home Secretary's decision "neither to confirm nor deny" (NCND) that Mr McGartland was an agent.
His case is the first of its kind.
Mrs May's stance has meant there has been no response to Mr McGartland's specific allegations.
James Eadie QC, for the Home Secretary, said in a written argument before the court: "An assurance of 'secrecy forever' lies at the heart of the relationship between the security service and its agents. The strict maintenance of the NCND principle is one of the most important means by which the Security Service makes good that assurance."
Mr Justice Mitting proposed a compromise in which the Home Secretary would agree to it being taken as proved, for the purposes of the court hearing only, that Mr McGartland was an agent, without her having to abandon NCND policy. The judge said this would enable the courts to deal with the central allegations. The court will hear today whether his plan has been accepted.