Cycle thefts in London have doubled in the past four years, according to the latest figures.
Campaigners say the rise could wreck plans for more cycling
More than 21,200 bikes were stolen in 2005/6 - double the number taken in 2001/2, said the Metropolitan Police.
Campaigners warn it could wreck plans to encourage more Londoners to travel by bike.
"The more cyclists we get the more this is going to happen unless police take it seriously," said Jenny Jones, Green Party Member at the London Assembly.