Police say victims of the attacks are often not aware of how cyber-hackers gained access to their bank accounts, and warn that the attacks are hugely under-reported.
Only 33 people have said they have lost money after being hacked while using a public wi-fi spot since 2010.
The alert was raised by Action Fraud, the agency run by City of London police which collates all reports of fraud in the UK.
There are now an estimated 269,000 free wi-fi points in the UK, compared to 202,944 in 2014, mostly used for e-mailing and social networking.
The fraud detectives warn that people using public wi-fi should avoid online banking, downloading apps, installing updates and online shopping, and try to check if a network is genuine.
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