Messages sent to around 200 IRA on the runs informed them that they were not wanted by police. The Government insists these did not constitute immunity from prosecution.
The Northern Ireland Office (NIO) scheme emerged during the collapse of the trial of alleged Hyde Park bomber John Downey after police mistakenly sent him one of the letters even though he was sought by the Metropolitan Police.
Julie Hambleton, whose sister Maxine died, said: "We are incandescent with frustration, anger and more grief. It is almost as if we are re-living the horrors of losing our sister all over again and being slapped in the face."
Paddy Hill received a life sentence but his conviction was later quashed.
His spokesman, Paul McLaughlin, project manager at the Glasgow-based Miscarriages of Justice Organisation, said he denied making comments attributed to Mr Hill which suggested that police sent secret letters to two of the men suspected of involvement in the pub bombings telling them they would not be prosecuted.
Nobody has been brought to justice for the killing of 21 people in Birmingham on November 21, 1974. Mr Hill was jailed along with Hugh Callaghan, Gerard Hunter, Richard McIlkenny, William Power and John Walker.
The Birmingham Six spent 16 years in prison before their convictions were quashed in 1991.