The head of Europol has warned against using free public wi-fi for sending sensitive information due to the risk of interception by hackers.
Troels Oerting, head of EU law enforcement agency's cybercrime centre, told the BBC that the public wi-fi provided in many cafes and shops is being targeted by 'man-in-the-middle' (MITM) style attacks.
"We have seen an increase in the misuse of wi-fi, in order to steal information, identity or passwords and money from the users who use public or insecure wi-fi connections," he said.
"We should teach users that they should not address sensitive information while being on an open insecure wi-fi internet.
"They should do this from home where they know actually the wi-fi and its security, but not if you are in a coffee shop somewhere you shouldn't access your bank or do all of these things that actually transfer very sensitive information."
The MITM attack involves tricking people into connecting to a separate wifi hotspot from which criminals can monitor information such as passwords being sent between the user and online shops, banks or social media sites.
"Everything that you send through the wi-fi is potentially at risk, and this is something that we need to be very concerned about both as individual users but also as police," he added.
The warning follows a recent man-in-the-middle attack on the European Parliament. Access to its public wi-fi was cut after an attack captured communications between private smartphones and the Parliament's own network.
MITM style attacks are not new, according to Carl Leonard, senior security research manager EMEA at IT security firm Websense, but are increasingly prevalent.
"The call by Troels Oerting, head of Europol's cybercrime centre, for people to not send personal data over public wi-fi hotspots whilst welcomed is not particularly unforeseen in the IT security community.
"The threat posed by compromised wi-fi hotspots and particularly MITM attacks has existed for years and, though it has not quite received mainstream notoriety, MITM instances always have the potential to seriously affect a multitude of industries in 2014."
Attackers will target all individuals, but business people working remotely at an airport, for example, are highest on the list.
"The nomadic businessperson, for example, is a prime target for those cybercriminals looking to intercept valuable data that can turn a lucrative profit. Cybercriminals are often targeting intellectual property and financial information so if a MITM attack is successful, organisations can experience negative brand perception, reduced customers' confidence and ultimately their bottom line."