"The number of car jamming incidents in the Johannesburg area has gone up in the past 18 months, and not much can be done to stop the jamming as car remotes are electronic transmitters," Aon South Africa spokesperson Kate Lellyett said in a statement.
She said exact numbers were not available, but quoted from a newspaper report that 40 cases a week had recently been reported in the Sandton area.
"It's a challenge to give you a scientific figure as there are a number of insurance products that don't cover this type of loss, so we would not have total reported claims to check against.
"However, on the policies that do cover it we have seen a marked increase, but because of the number of unreported cases it's a challenge to quantify total losses as result," she said.
Car jamming involves blocking car remotes using a household remote because both remotes operate on a 400Mhz frequency. Criminals prevent the locking action of the car from being activated.
Lellyett said the real defence was to mitigate risk by being aware of the practice and personally checking that car doors were locked.
"Make sure you hear the beep of your alarm system and the audible sound of the locking mechanism.
"Physically check your doors and boot... Your valuables should be kept in your boot and out of sight," she said.
"Jamming is being executed by professional gangs and motorists will need to remain vigilant to prevent falling victim to this scam."
On Wednesday morning, The Star newspaper reported that of the 40 cases of theft from vehicles reported in Sandton each week, at least 85% were thefts in which jamming took place.
Aon SA said that with the average cost of a laptop at R10 000 and a lady's handbag at R5 000, the costs were substantial, with losses of around R510 000 a week and more than R2m a month.
Car jamming had significant implications, Lellyett said.
"The reality is that unless there are signs of forcible or violent entry, most insurance policies won't pay out for claims of theft of valuables from your vehicle."