The note asked for a wide-ranging inquiry which seeks to discover what could have been done to prevent the London attacks, which killed 52 people.
Calls for an investigation come after it emerged yesterday that MI5 observed two of the London bombers - Mohammed Sidique Khan and Shehzad Tanweer - meeting with the now jailed ringleader of the fertiliser plot.
Jacqui Putnam, who survived the Edgware bomb attack, questioned what else the survivors had not been told while another survivor, Rachael North, said it was an "opportune moment" for an inquiry.
Ms North, who was on the Piccadilly line train which was attacked, said: "This is an opportune moment to reassert the case because the British public are far more aware of the situation now."
The letter does not specify any connection to the information released yesterday, but does attempt to draw attention to any measures which could have been followed to possibly prevent the London attacks.
It says that an inquiry could be used to "examine issues aimed at saving lives, minimising suffering and improving the response of government agencies to the continuing threat of terrorist attacks".
Mr Reid ruled out a public inquiry for a number of reasons, including not wanting to use up valuable resources which are already stretched in dealing with ongoing threats.
In addition, three people are currently awaiting trial in relation to the July 7th bombings which is the principal reason for the objection to an inquiry.
Yesterday, Omar Khyam, 25, of Crawley, West Sussex, who was described as the fertiliser plot's ringleader, was found guilty of conspiring to cause explosions likely to endanger life between January 1st 2003 and March 31st 2004 at the Old Bailey.
Waheed Mahmood, 34, and Jawad Akbar, 23, also of Crawley; Salahuddin Amin, 31, of Luton, Bedfordshire; and Anthony Garcia, 24, of Barkingside, east London, were also found guilty.
All five were sentenced to life in jail.
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