The warning comes after one fleet had to change the kits on hundreds of vehicles after a mobile phone supplier refused to install them because they did not have external aerials.
Kaba Door Systems ordered more than 250 kits from Orange but the mobile phone provider refused to fit some of them as it was feared that some phones with internal antennae could affect the cars' electronics and increase levels of radiation inside the car.
Ann Dukanovic, fleet manager at the company, explained: 'We signed a new contract with Orange for 250 phones, all Nokia consisting of 6310s, 7250s, and 3510s. When we tried to order car kits for these we were told they would only fit the ones for the 6310s as the others had a health and safety issue.
'This has cost our company a lot of money in replacing handsets and also in reinstalling new kits. We were also left with a load of nearly-new phones.'
Two types of car kits are available: actively coupled and passively coupled. Active kits have an external aerial socket whereas passive kits have no antenna so radio waves are not passed outside the vehicle.
Orange confirms that it did have a problem installing the kits. A spokesman said: 'There were a couple of issues over why we temporarily did not fit passive car kits.
'Some car manufacturer warranties stipulate that there must be no interference with the car's electrics and there was a possibility that these kits could affect the electrics. There was also a slight element that the kits could increase the radio frequency inside the cabin despite the output for these phones being well below international standards. But the key concern was invalidating the warranty.
'We now supply and fit these kits and have been doing so for the past six months.
'We have had discussions with manufacturers but we do insist that fleets check their manufacturer's warranty before installing kits.'