Families of those killed in the July 7 attacks will learn whether the inquests into their loved ones' deaths will investigate alleged failings by MI5.
The coroner, Lady Justice Hallett, will announce the scope and format of the upcoming inquests in a hearing at the Royal Courts of Justice in London.
Key issues she will rule on include whether the inquests for the four suicide bombers should be held jointly with those for their 52 innocent victims.
Lawyers for many of the bereaved families say the hearings should include a broad-ranging investigation of whether the authorities could have prevented the 2005 London bombings.
They want to use the inquests to ask MI5 officials why they did not follow up plot ringleader Mohammed Sidique Khan after he was witnessed meeting known terror suspects 17 months before the attacks, but the security service argues this is both unnecessary and impossible because doing so would require the disclosure of top secret intelligence files.
Counter-terrorism officers watched, photographed and followed Khan in early 2004 during an inquiry into a group of extremists planning a fertiliser bomb attack but MI5 concluded that diverting resources to place him under detailed investigation or surveillance was not justified.
Neil Garnham QC, counsel for the Home Secretary and MI5, told a pre-inquest hearing that revealing why the security agencies failed to investigate Khan could give al Qaida terrorists an "invaluable weapon".
It was disclosed last month that West Yorkshire Police have only just discovered that they held Khan's fingerprints on file for 19 years before the attacks.
Other issues the bereaved relatives want the inquests to examine include whether the emergency services could have saved more lives. The family of hospital biomedical officer Behnaz Mozakka, 47, assumed she died instantly but learnt last month that she survived for up to 40 minutes and even spoke to a police officer after the blast.
Lady Justice Hallett, a senior judge in the Court of Appeal appointed as coroner, will also rule on whether she should sit with a jury and she must decide who should be counted as "interested persons" entitled to be represented at the inquests. The bereaved relatives are automatically classed as interested persons, but some survivors of the attacks argue they should also have the status.