Fake Sutton Council workers stole hundreds of pounds from a 97-year-old woman after claiming they needed to check her home for water leaks.
Three men called at the pensioner's Brunswick Road flat on Saturday afternoon and pretended to be council staff - but showed no identification.
They asked the woman and her daughter to fill saucepans from the kitchen tap while one of them stole £300 cash from a bedroom, Sutton police said.
Police urged residents not to let doorstep callers into their homes. Anyone with information about the burglary should call police on 101.
To find a registered, reputable trader, check Sutton's trader scheme at sutton.gov.uk/ssts or call 020 8770 5070.
For advice on rogue traders call Citizens Advice on 08454 04 05 06 or visit citizensadvice.org.uk/index/consumer_service.
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