A report found the radio system used by London ambulance workers failed during heavy rain, it has emerged.
During the Health and Safety Executive inspection crews also revealed it was not uncommon to go out without radios.
The digital Airwave system was brought forward after communication during the 7 July bombings was criticised.
The London Ambulance Service (LAS) said its staff were not expected to go out without radios and it had no evidence of problems in rain.
The March HSE inspection report showed crews had reported some problems with the use of radios, namely poor reception during heavy rain.
Crews also said panic buttons either did not not work or elicited no response when activated, it found.
The report said this was "potentially worrying" and, if true, could lead to the development of a false sense of security.
At least one crew interviewed was found not to have radios and it was stated that this was not uncommon.
The report made a number of recommendations including updating procedure to reflect the introduction of radios.
It said issues raised by staff about the operational effectiveness of radios should be investigated and appropriate action taken.
It also said a system should be put in place to ensure staff could not leave ambulance stations without radios in the vehicles.
The LAS said its vehicles were regularly checked to ensure they had the correct number of radios and it had found "no evidence of problems with Airwave radios during heavy rain".
In a statement it said: "We have fully investigated the points raised about our Airwave radio systems in this report and followed the recommendations made.
"The safety of our staff is of the highest importance to us and we are committed to giving our technical teams all the support they need to ensure that staff and vehicles have robust systems in place to ensure they have fully functional radio communications."