Campaigners for victims of the Birmingham pub bombings will meet Home Secretary Theresa May this week to press for a new inquest into the deaths.
Bereaved relatives will also challenge the Government on claims a disclosure embargo on confidential state files related to the 1974 attacks has been extended to 75 years.
Twenty one people were killed and 182 injured when the suspected IRA bombs exploded in two city centre pubs on November 21, 1974.
Six men wrongly convicted of the murders - the Birmingham Six - were released in 1991 after their convictions were overturned by the Court of Appeal.
An inquest opened days after the bombings was closed without hearing evidence in 1975 in response to the guilty verdicts.
Campaign group Justice 4 the 21, which represents relatives of a number of the victims, are calling for a new inquest to be ordered.
An application requesting a fresh probe has been lodged with Attorney General Jeremy Wright.
Relatives will ask for the support of Mrs May and Northern Ireland Secretary Theresa Villiers at a meeting at the Home Office in London on Wednesday.
Julie Hambleton, whose 18-year-old sister Maxine was killed, said the 21 victims had a right to a proper inquest.
"What completely stumps us is the fact when the men were released, when their convictions were quashed, nobody stood up and said 'right, we've got to go back through the motions, we need to have a coroner's inquest, we need to have another investigation, we need to go above and beyond and go and look for the murderers'. Nobody did that," she said.
In relation to the release of state files, Ms Hambleton added: "If they have got nothing to hide why have they buried the evidence for 75 years? Why are they waiting for us and the murderers to die before they allow these papers out? What is it they've got to hide? If they've got nothing to hide then why not release the papers to help the families finally get to the truth."
She added: "We want all the support we can get and we want the Government to take the stance that they fully support our campaign for truth and justice."
The campaign group is being assisted by Belfast-based solicitor Kevin Winters.
"For the victims of the Birmingham Pub Bombings in 1974 the purpose of this meeting is to secure a new inquest into the deaths of their loved ones, including into the cause of their murder and whether the bombings could have been prevented and to identify the failures in the police investigation following the bombings which resulted in the miscarriage of justice of the Birmingham Six," said Mr Winters.
"The meeting will also be an opportunity to request that the Government's 75 year ban on disclosure of relevant material be lifted and for it to recommend a human rights compliant inquest with publicly funded victim participation in proceedings and disclosure from all the relevant interested parties and persons."