A lack of mobile phones and a breakdown in the London Ambulance Service's radio system made dealing with 7 July bomb victims difficult, a review has heard.
The London Assembly heard ambulance crews were therefore unaware of the scale of the disaster and which hospitals to take casualties to.
Just 10-15% of radio traffic was getting through on the day.
As a result, the Royal London and Barts hospitals got a high number of patients but Newham and Homerton got few.
The review was also told that the Great Ormond Street Hospital (GOSH), situated near the Tavistock Square bus bomb, was not involved in emergency planning.
"You really don't know what's going on - all you can rely on is that the hospital that you've last driven from is still intact"
This meant it was not informed of developments and had to send runners to the bomb sites.
Eventually it set up its own field hospital to help ambulance crews who could not get through to the hospitals which had been put on standby.
Judith Ellis, a chief nurse at GOSH, urged the NHS to rethink their communications policy in a major emergency.
Dr Gareth Davies of the Barts and The London NHS Trust said there was a sense of "Armageddon" because information was not being passed on.
"You really don't know what's going on. All you can rely on is that the hospital that you've last driven from is still intact," he told the review.
Some 52 people were killed in the London bomb attacks on 7 July.
The review was adjourned until 22 February when survivors will be among the witnesses.
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