Survivors of the 7 July London bombings have begun legal proceedings over the government's refusal to grant an independent public inquiry.
The attacks on London's transport system killed 52 people
The group of survivors and relatives of those killed has applied for a judicial review of a Home Office decision not to hold an inquiry into the attacks.
The Home Office said it had received the legal papers but would not comment before giving them full consideration.
Four suicide bombers killed 52 people and injured nearly 800 in 2005.
The group of survivors and relatives of the dead had said they preferred not to pursue a formal judicial challenge and incur potentially costly litigation.
'Wider public interest'
But Oury Clark Solicitors, which represents the group, said it had launched the action because the government had not "adequately engaged" with them and their clients.
They say the Home Office failed to answer a letter by the deadline of 24 August, instead saying it would respond on 7 September.
But judicial review proceedings had to be applied for by 30 August, they added.
James Oury, senior partner of Oury Clark Solicitors, said: "The government's position has resulted in a group of still-suffering victims of this public tragedy being manoeuvred into a litigation arena so not to be further disadvantaged.
"Our clients are disappointed and saddened by this. Their obviously preferred position was not to enter into litigation."
But the survivors recognised "the wider public interest involved and the international importance of finding out the truth of events leading up to these...bombings", he said.
The legal action was being taken to ensure lessons were learned for the future and legal proceedings would be stopped if an independent and public inquiry was launched, he added.
The group says an inquiry is necessary to allow public scrutiny of events and to enable the families of those killed, survivors and other agencies to be involved.
BBC home affairs correspondent Daniel Sandford said that although there had been an investigation into the bombings by the Intelligence and Scrutiny Committee and a government narrative of events, the group felt they had been inadequate.
He said both took place before May's fertiliser bomb plot trial which revealed that security services had watched and followed two of the 7 July bombers, Mohammad Sidique Khan and Shehzad Tanweer, a year before the attacks.
The survivors and the families of those who died argue that the government's refusal to hold an inquiry breaches the Human Rights Act because it is failing in its duty to protect life, our correspondent added.
But, he said, the government was against holding such an inquiry because it said it would be a drain on resources and tie up key officials and police officers.