A request by the Manchester Arena bomber's elder brother to be given immunity in return for speaking to the attack inquiry has been denied.
Ismail Abedi, 27, from Manchester, had refused to answer any of the inquiry's questions and claimed a general privilege against self-incrimination.
An immunity application was made to the inquiry's chairman Sir John Saunders.
Refusing it, Sir John said any failure to prosecute may be seen as "a considerable affront to justice".
Twenty-two people were killed and hundreds were injured on 22 May 2017 when Salman Abedi detonated a bomb at the end of an Ariana Grande concert.
The bomber's younger brother Hashem Abedi, who was part of the conspiracy, was jailed in 2020 after being convicted of murdering all those who died.
The application by Ismail Abedi, who was arrested under the Terrorism Act the morning after the bombing but later released under investigation, had been opposed by lawyers representing bereaved families.
He has refused to answer the 40 questions posed to him in writing by the inquiry or agree to appear in person as a witness.
Some witnesses in other public inquiries have received assurances that their evidence will not be used against them in a criminal prosecution.
In his ruling, Sir John said allowing immunity to get Mr Abedi to co-operate did not outweigh the potential effects on the administration of justice.
"If as a result of an undertaking from the Attorney General, the applicant was to disclose material to the inquiry which provided evidence to justify charges of murder or conspiracy to murder, then he could avoid trial for 22 murders and causing serious injury to many more," he said.
"While less serious, if he were to disclose material... which evidenced a failure by him to disclose information to the authorities which could have prevented the bombing happening, a failure to prosecute would be considered by many to be a considerable affront to justice."
Mr Abedi is expected to be served with a legal notice soon which will "require" his attendance in person at the inquiry and while he will be entitled not to answer questions on the grounds of self-incrimination, he will have to justify why and on what grounds he is refusing to answer.