Porsches, BMWs and Jaguars have so far been recovered

About 100 officers raided addresses in London, Bedfordshire and Hertfordshire early on Thursday.

They were part of an operation which began in October to stop the rising number of car thefts following burglaries.

Police say burglars spot expensive cars parked on driveways and then break into the owners house, steal the keys and take the car.

In the north London borough of Barnet alone, there have been 131 burglaries where cars worth at least �25,000 each were stolen.

Stun gun seized

Cars worth a total of �7m were stolen during 169 burglaries in Hertfordshire.

A Scotland Yard spokesman said: "Residents have unwittingly left car keys in view from windows or in insecure places where they can be easily removed by the burglars."

Eight men and three women were arrested during the raids on suspicion of various crimes including conspiracy to burgle.

Two cars, a stun gun and a quantity of drugs, believed to be cannabis, were also seized.

Since the operation began last October, 23 cars worth a total of �540,000 have been recovered.

Sixteen are being examined by forensic experts at a high-security pound.