More than 1,200 bicycles are stolen every day in the UK, according to research by insurance firm Direct Line.
Even bicycles that are chained up can be taken by thieves
The company left locked bikes in ten cities around the country and most were stolen within 24 hours - half disappeared within two hours.
The company's study suggests 440,000 bikes are stolen by thieves each year.
Consumers are being warned that they could be fuelling the problem by not checking ownership of bicycles bought second-hand.
Almost half of second-hand bikes are now bought over the internet.
Direct Line spokesman Andrew Lowe said: "Our experiment demonstrated just how quickly a bike can be taken even when locked up in a public area."
As part of the study, a bike left in Croydon, south London, was stolen within an hour.
Bikes locked up in the centres of Manchester and Liverpool were taken within two hours.