Two MI5 agents whose testimony may throw light on Mr Martin McGuinness's movements on Bloody Sunday will have to give "live" evidence to the inquiry, it has emerged.
They are being called because they worked with a former IRA informant who claimed that Mr McGuinness, the Sinn Féin MP for Mid-Ulster, fired the first shot in Derry on January 30th 1972, when British paratroopers killed 13 men after a civil rights march.
The inquiry has rejected an application by the security services for a time-delay restriction to be imposed when the agents, identified only as Officers A and B, take the stand.
The rejected application also included former MI5 agent David Shayler, an inquiry spokesman confirmed.
Officer A acted as a handler for the informant, codenamed Infliction, while Officer B debriefed him.
Lord Saville, the inquiry chairman, has previously ruled that Infliction will not have to give evidence because it would put his life at risk and breach his human rights.
Infliction is alleged to have told his handlers that he heard Mr McGuinness confess to firing the first shot. Mr McGuinness has admitted being the IRA's number two in Derry on Bloody Sunday, but he insists that he did not fire shots.
The security services had asked that only the tribunal panel, counsel to the tribunal and security service representatives be in the inquiry chamber when the men testify and for transcripts of their evidence to be delayed and checked before it is released.
They suggested that members of the public, including bereaved relatives and legal representatives of the interested parties, should be cleared from the chamber when the agents give evidence.
Lawyers representing the victims' families should write a list of questions which would be asked in their absence.
The tribunal said that it "recognised the difficulties regarding the inadvertent release of potentially damaging information" in questioning these officers in public, but it maintained that "adequate" safeguard procedures would be in place.
Several security service officers will be screened from view while they give evidence to the inquiry, which is currently sitting in London for security reasons while hearing evidence from military and political witnesses.
No dates have been set for the officers to appear before the tribunal.
It is unclear whether it will be necessary to call Mr Shayler who, like other witnesses, has provided a sworn statement, to give evidence in person, the spokesman said.
In coming to its ruling, the tribunal panel read a number of confidential documents relating to the operational integrity of the security service and its members in private.