Survivors of the July 7 terror attacks and relatives of those killed are due to take their calls for a public inquiry to the Home Office.
A letter addressed to John Reid and calling for a wide-ranging inquiry will be handed in at the department.
The move by the 7/7 Inquiry Group comes a day after it was revealed that MI5 had come across ringleader Mohammed Sidique Khan and his right-hand man Shehzad Tanweer while investigating another group of plotters.
The revelation during the fertiliser bomb plot trial at the Old Bailey prompted questions over the accuracy of some of the information families and survivors of the 7/7 attacks had been given.
Jacqui Putnam, a survivor of the Edgware bomb, said she had been told the bombers were acting alone but that what emerged from the fertiliser trial appeared to undermine that.
"What else have we not been told?" she said.
Survivors and families said a public inquiry could help establish what steps, if any, could have been taken to prevent the 2005 attacks.
"This is an opportune moment to reassert the case because the British public are far more aware of the situation now," Rachel North, who survived the blast on the Piccadilly Line train, said.
The letter, signed by people personally affected by the bombings, makes no specific reference to the trial's revelation. But it calls for a "comprehensive, accurate and definitive" account of how the bombings came about and the subsequent police investigation.
Mr Reid said on Monday he was "perfectly open to an inquiry" into the Security Service's handling of intelligence about Khan and Tanweer, but a public inquiry could not be held until after the trial of three people in connection with the July 7 attacks.
