A "very senior" MI5 officer will give evidence anonymously at the inquest into the 7/7 London bombings following a ruling by the coroner today.
Lady Justice Hallett granted an application by Home Secretary Theresa May to protect the identity of the witness.
But, although she ordered anonymity, the coroner refused to rule that Witness G should be screened from the bereaved families.
The witness is to give evidence on the question of whether the attacks could have been prevented.
Giving her decision at the Royal Courts of Justice in London, Lady Hallett said: "The bereaved families have been waiting over five years to see this witness or a witness from the security service give evidence.
"The issue of preventability is exceedingly important to them. It has been at the heart of most of their submissions to me ever since my appointment as coroner."
She said she was confident that it would make a "considerable difference" to the bereaved families to be able to see the witness give evidence "rather than hear the evidence come from a disembodied voice".
The attacks launched on July 7 2005 by Hasib Hussain, Mohammed Sidique Khan, 30, Shehzad Tanweer, 22, and Jermaine Lindsay, 19, were the worst single terrorist atrocity on British soil.
As well as killing themselves and 52 others, the bombers injured more than 700 people.
The inquest is expected to last until March.
The Home Secretary's application was for anonymity and also for the witness to give evidence behind screens - unseen by everyone but the coroner, security-cleared staff to the inquest, counsel to the inquest and the legal representatives of the "interested persons".
Lady Hallett said the Home Secretary "believes, on advice, that the witness should be protected in this way" because revealing his or her identity "would materially increase the risk to their safety".
It was also said that there would be "a significant threat to national security, given the nature of the operations in which witness G has been involved".
The witness was described by the coroner as "a very senior member of staff at MI5".
Lady Hallett said the media and bereaved families had "very properly accepted that this is an exceptional situation, that there is the possibility of a very real threat of material increase in the risk to the very life of the witness, and therefore they have accepted that an order for anonymity should be made".
Turning to objections to the witness being screened from the "interested persons", Lady Hallett said that "to date the bereaved families have acted with enormous dignity and moderation".
She had not seen the "slightest hint" they would wish to see anyone's life put in danger - their aim was "to save lives".
Lady Hallett said: "If I were persuaded that there was any increased risk to Witness G by refusing to allow him or her to give evidence from behind screens, I would not hesitate to grant the application.
"However, to my mind, provided sufficient protective measures are put in place, there should be no increased risk to the witness, other than that which would attach in any event to the witness giving evidence, albeit anonymously."
She said she would put in place as many protective measures "as I think reasonable".