According to research released this week, 80 per cent of motorists are leaving expensive possessions such as mobile phones, clothes and even their wallets in their cars.
Insurer LV, which conducted the research, has said that this carelessness is the reason why one in four drivers has been the victim of a break in.
The most-stolen items from cars are detachable stereos, CDs, money, clothes and satnav systems.
LV spokeswoman Emma Holyer said: "These figures show a casual attitude amongst motorists when it comes to leaving their belongings in their car.
"The fact is more car break-ins are by opportunist thieves who would probably not bother to break into the car if it looked as though it didn't contain anything to steal."
Despite the perceived risk, only one in ten break-ins occurred in a commercial car park, compared with the 40 per cent which happened when the car was parked outside the owner's home.
Men were more likely to have expensive possessions in their cars, with an average of £380 worth of valuables left in their cars in contrast to the national average of £283.
According to the AA, car insurance premiums are set to rise further after an increase of £50 since last year.
It estimates that the average premium is now £724.28.
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