A disc containing personal information have been lost by the public inquiry team probing the loyalist murder of a Catholic solicitor in the North.
Investigators looking into the death of Rosemary Nelson, 40, said a police investigation had been launched but the material was not believed to have been stolen.
Mrs Nelson died after a booby-trap bomb left by loyalists exploded under her car in March 1999.
A spokesperson for the panel said: "The inquiry deeply regrets this serious breach of secure data handling protocols within the inquiry.
"Immediate steps have been taken to avoid any recurrence and a comprehensive review of all aspects of data handling has been initiated.
"The inquiry is determined that the incident should not in any way compromise or impede its investigation into the circumstances of the death of Rosemary Nelson."
The compact disc went missing on May 6.
The spokesperson added: "The inquiry has been advised that there are currently no grounds to suspect that there will be any adverse consequences as a result of this loss.
"Given the nature of the information, it would not be appropriate to say more about the material involved."
Retired judge Michael Morland is chairing a three-strong panel examining alleged security force collusion in the murder by the Red Hand Defenders (RHD).
The inquiry must determine whether the Royal Ulster Constabulary (RUC), Northern Ireland Office (NIO), army or other state body allowed the murder, or blocked attempts to investigate it.
The collusion allegations followed Mrs Nelson's role as the legal representative in a number of high profile cases, including that of the nationalist Garvaghy Road Residents' Coalition in Portadown.
The inquiry, which opened in 2005 to set out its terms of reference, has already gathered tens of thousands of documents.
Northern Secretary Shaun Woodward said: "The government has made clear to the Inquiry its grave concerns about the breach of the detailed agreement it has with the Inquiry on the handling of data.
"As soon as we were made aware of the loss, the government instituted all necessary steps to meet its obligations."
"While the handling of data in its possession is entirely a matter for the independent inquiry, I have initiated an urgent and comprehensive review to ensure that the inquiry is complying fully with the procedures the government agreed with them."
He said his department had written to the North's four public inquiries asking to reinforce the need to comply with all data security procedures.
The others involve the 1972 Bloody Sunday inquiry into the shooting dead of 14 civil rights protestors in Derry by soldiers, the beating to death of Catholic Robert Hamill, 25, in Portadown, Co Armagh, in 1997 by a loyalist mob while police allegedly looked on, and the death of former Loyalist Volunteer Force head Billy Wright in the high-security Maze Prison at the hands of armed splinter republican prisoners.
Earlier this year a laptop from the Wright inquiry containing confidential information was stolen.
The computer, belonging to a barrister who represents prison service staff in the inquiry, contained the details of a number of people in the North.