Families of victims of the 7/7 suicide bomb attacks in London will learn later whether M15 officers will be called to give inquest evidence.
Lady Justice Hallett, who is acting as coroner, will announce the scope of the inquests, due to start in October.
Relatives want the inquests to examine whether the attacks in July 2005 could have been prevented.
The inquests have been delayed by court cases linked to the attacks in which 52 innocent people were killed.
Surveillance operation
With the fifth anniversary of the 7/7 bombings approaching, the bereaved have had a prolonged wait for the inquests.
Friends of the bombers were tried and acquitted of any involvement in the blasts.
Two of the bombers had cropped up in a surveillance operation more than a year before the attacks.
BBC home affairs correspondent June Kelly says this is why the families want the role of the police and MI5 to be examined as part of the inquests.
The families want the inquests to ask why the security service did not investigate the bombers’ ringleader Mohammed Sidique Khan further.
MI5 has said this evidence would give al-Qaeda an "invaluable weapon" and should not be disclosed.
Survivors decision
Lady Justice Hallett will rule on this and a number of other issues in a hearing at the Royal Courts of Justice in London.
The judge has been asked to decide whether the deaths of the 52 innocent victims of the blasts should be considered at the same time as those of the four suicide bombers.
Relatives of those who were killed are opposed to this, saying it would cause too much distress.
And she will rule on whether 7/7 survivors should be designated as interested persons, therefore having a voice at the hearings.
The suicide bombings took place in central London on Tube trains at Aldgate, Edgware Road and between King’s Cross and Russell Square, and on a bus at Tavistock Square.
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