The report, by the 7 July Review Committee of the elected London Assembly, said some rescuers relied on radios that did not work on the underground rail network and others' mobile phones failed.
The report said: "This breakdown in communication led to a failure to deploy the right numbers of ambulances to the right locations; a lack of necessary equipment and supplies at the scene and delays in getting some of the injured to hospital.
"It is unacceptable that the emergency services, with the exception of the British Transport Police, are still not able to communicate when they are underground."
The most striking failing was the lack of planning to care for people who survived and were traumatised, the report said.
Four suicide bombers killed 52 commuters and injured 700 in attacks on three trains and a bus in the capital during the morning rush hour on July 7, 2005.
Three men detonated bombs on separate underground trains shortly before 9am, and a fourth blew himself up on the bus in central London about an hour later.
Richard Barnes, chairman of the committee that wrote the report, said police, fire and ambulance crews had been "incredibly brave" but that lessons needed to be learnt after their communications broke down.
An over-reliance on mobile phones was exposed when police in the City of London financial district ordered the temporary shutdown of part of the network without checking with those leading the response.
"It is unacceptable that the emergency services ... are still not able to communicate when they are underground.”
London Assembly report
"We actually had someone acting outside the command structure on July 7 and that was not necessarily helpful," Barnes told BBC Radio.
The failure to provide survivors with a central place to gather after the bombings to get help and give their details was another big mistake, he said.
The London Assembly set up a cross-party review body in September 2005 to examine the lessons to be learnt.
Ken Livingstone, the mayor of London, said the emergency response "worked virtually perfectly", but witnesses exposed serious failings during the inquiry.
In March, survivors said some emergency crews were slow to respond to the explosion on a train at Aldgate, in the City of London financial district, over fears of a secondary blast.
One passenger, named only as Michael, said he walked past police and fire crews on the platform as commuters lay dying in a nearby tunnel.
The Report of the 7 July Review Committee is available as a text and PDF file at this page on the London Assembly site