City of London police called for the mobile phone network to be shutdown when London was bombed, contrary to the agreed procedure, the BBC has learned.
Many were unable to get through to loved ones on their mobiles
The force acted independently to stop mobiles working for four hours within a one-mile radius of Aldgate Tube station, one of the four sites hit.
The London Assembly findings come despite the Metropolitan Police (Met) denying the network was closed down.
The government is to carry out its own investigation into the matter.
After the 7 July bombs, many Londoners were anxious to call family and friends to let them know they were safe, but were unsuccessful and told it was due to a systems overload.
The Met said it had rejected an option to switch off the network which would have allowed access only to emergency service staff with special sim cards.
Speaking last month, Met Commissioner Chris Allason said: "The decision between all of us was - no we are quite happy. We are content we have got command and control through our radio systems."
But on Thursday, it emerged a senior City of London officer had made the decision, despite being part of the Gold Command team which had decided against the move.
A Met spokeswoman said: "Any such closure was unilateral and not communicated or authorised by the command group."
David Sutton, of O2, told the London Assembly the decision had caused confusion.
"On the day, for Gold Command to have a different view from one of the forces was not a satisfactory conclusion."
Richard Barnes, Conservative London Assembly member, said: "That the City Police, a local police service, could make a decision outside that chain of command to close down the mobile service... I find it extraordinary."
City of London police, however, told BBC News the decision was made by the most senior officer and they would do the same again if they needed to retain communication.