There are a high number of thefts from cars in Richmond
It is not uncommon to find items stolen from unlocked cars but it is rare that the "culprit" is a police officer.
Officers in Richmond, south-west London, are entering unlocked cars to remove valuables, such as handbags and laptops, leaving a note for the driver.
Supt Jim Davis said no law was being breached but admitted: "Technically we are entering the vehicle."
But a leading lawyer said police could face civil proceedings for trespass if any items taken by them were damaged.
The new police tactic, launched last month, is aimed at tackling thefts from cars which has recently risen by 40% in the area.
Until now, 25 cars have been targeted and an item has been removed in only one case. Supt Davis said Richmond Police had received "no negative complaints" so far.
But lawyer Orlando Pownall QC, who lives in Richmond Hill, said: "It undoubtedly is a trespass and there could be civil proceedings if a person shows a damage or loss has resulted from the trespass.
"Police need to ensure that information on a computer or mobile is not looked at and kept in a secure place - otherwise it could be an intrusion.
We don't want to take people's property as it is an awful lot of bureaucracy and hassle for us
"It might also be worthwhile for officers to take a photo like traffic wardens."
Supt Davis said officers on patrol were only targeting unlocked cars with a valuable item on display.
Officers try to contact the owner and if that is not possible they remove the item and leave a note for the driver advising to retrieve it from Twickenham police station.
In 2008, 1,300 thefts from cars were reported in the borough, of which 220 involved sat-navs. But the figure had risen by 40% by this July.
Supt Davis said: "We identified that 21% of thefts from cars were from unlocked cars, so no sign of forced entry.
"Technically we are entering the vehicle but we are not committing a crime. It's a common law duty to protect (people's) property.
"We don't want to take people's property as it is an awful lot of bureaucracy and hassle for us but we are doing this to make sure people take responsibility of their valuables."
Richmond Council said it backed the police's scheme.