Two judges in London dismissed the challenge by Hasina Patel, whose husband, from Dewsbury, masterminded the plot.
She was criticised by one of the judges who said she had not shown any willingness herself to help the inquest proceedings.
It had been argued on Ms Patel's behalf at a hearing earlier this week that the decision to deny her legal aid was "unfair, irrational and unlawful".
But Lord Justice Thomas and Mr Justice Silber said the decision by the Lord Chancellor "cannot be described as unreasonable or irrational".
Lord Justice Thomas said the court was told that Ms Patel's position "was that she was interested to understand why her late husband and the other bombers acted as they did" and that what she was seeking "was an opportunity to ask questions of witnesses at the inquest which bore on their knowledge and experience of her husband and others".
The judge said: "Far from providing any information that might assist the wider public interest, she has flatly and unequivocally declined the opportunity to do so.
"Although requested by this court to show how she could help establish why her late husband and the others whom she knew acted to murder fellow citizens, she has provided not an iota of evidence to us which could show how in some way she could bring a wider benefit let alone a significant benefit to the inquests or to the understanding of the victims."
It emerged in April that Ministers had rejected two applications for legal aid by relatives of the bombers after ruling they did not meet the criteria for public funding for their lawyers.
Legal aid will be offered to the families of the four 7/7 bombers' innocent victims of the attacks.
As well as killing themselves and 52 others, the bombers – Khan, 30, Hasib Hussain, 18, Shehzad Tanweer, 22, and Jermaine Lindsay, 19 – injured more than 700 people.