Historic paving stones are being ripped up from London's streets at night and sold on by thieves for thousands of pounds.
Patching up the pavements has cost Camden Council nearly £30,000
There have been 24 thefts in north London so far this year, largely from the pavements of Hampstead and Camden.
It is believed the thefts are being fuelled by the boom in garden makeovers and the stones are ending up in people's driveways and patios.
Camden Council, which has to patch up the gaps, says the thefts of York stones have already cost them nearly £30,000.
The authority is now offering a £500 reward to try to catch those responsible and is asking residents to be extra vigilant.
Martin Reading, from Camden Council, said: "We feel that they may be being used by contractors for patios and driveways.
"If you are having a driveway or patio reconstructed at the moment, ask the contractor you are using where they got the stones from and, even better, ask them to see the receipts."
The council is not the only victim. Thieves ripped up stones worth about £5,000 from around a church war memorial at All Hallows Church, in Gospel Oak.
Father David Houlding said he noticed the stones were missing when he arrived at the church one morning.
He said: "I come out almost every day now to have a look, just to see if any more stones have gone. It's really frustrating."