More than 50 manhole covers have been stolen in the space of a week in one borough in south London.
"Gaping holes" had been left in roads in Sutton, a council spokesman said.
About £2,500 has been spent on plastic covers to replace missing metal ones weighing up to 80kg (176lbs). These can be sold for scrap at £100 a tonne.
The "irresponsible" crime risked the lives of road users and pedestrians, said Councillor Simon Wales, Sutton's cabinet member for transport.
"It is also costing councils thousands of pounds at a time when cash is desperately short."
Police have been liaising with local scrap-metal merchants to tell them about the problem.
And they asked anyone who saw a manhole cover being lifted on to a truck or into a van to note the registration number and dial 999.