This kind of fraud has been happening for more than a year, but recently appears to have increased in intensity with hackers taking to the dark web to seek more victims.
Colleen Marples, 44, from the Derbyshire Dales, lost £147 when booking a holiday in Egypt for her husband's 50th birthday in March.
"I believe, as a customer that chooses to use the official platform set up by the company, you can expect a level of security and trust from within that system."
Meanwhile, Ian Robinson, 64, from Cumbria, described how hackers attempted to scam him twice for £122 and then £283 at two unrelated hotels in separate towns, as he booked a road trip in the UK.
A spokesman for Booking.com said that there was no "silver bullet to eradicate all fraud on the internet" but that the company's security team were always monitoring and stopping new threats.
The company said it was also monitoring for suspicious activity on its app and disabling links being shared if the chats appear illegitimate.
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