The Home Secretary is trying to prevent secret evidence at the 7 July terror attack inquests from being heard in public.
Theresa May is seeking a judicial review of the coroner's ruling in favour of the bereaved families, who want to see the evidence and question the security service, MI5, about the intelligence gathered before the bombings on London's transport network in the summer of 2005.
Last week, Lady Justice Hallett rejected arguments by lawyers for MI5, who argued that she had powers to exclude victims' families from the hearing while she was examining the highly sensitive documents. She said the evidence could be edited to remove names of sources and other confidential information which might threaten national security. Yesterday, the Home Office said it would appeal against her decision. It said that while it welcomed the inquests, they did not mean "that we will put lives at risk and undermine our national security by not protecting sensitive material".
Families and survivors won a victory in May when Lady Hallett, an Appeal Court judge who was appointed assistant deputy coroner to hear the inquest into the deaths of the bombing victims, said the hearings should examine alleged failings by MI5 and the police.
Relatives want to ask security officials why they did not pursue Mohammed Sidique Khan, the ringleader of the plot, and his right-hand man, Shehzad Tanweer, after surveillance officers watched them meeting known terror suspects 17 months before the bombings.