Incidents of online ticket fraud rose by 55% last year to a cost of £5.2 million to the UK public, figures show.
The cost is a leap from the £3.35 million lost by consumers in 2014, with more than a quarter (26%) of fraudulent tickets sold online last year for Rugby World Cup and Premier League matches, according to the Government-backed web security initiative Get Safe Online and the City of London Police's National Fraud Intelligence Bureau (NFIB).
Some 15% related to fraudulent tickets to gigs and festivals.
Those aged 20 to 29 accounted for more than a quarter (28%) of victims last year.
Get Safe Online urged sports and music fans to be vigilant, especially on social media sites, ahead of the Uefa Euro 2016 Championship in June and the summer festival season.
Findings show that 21% of crimes relating to ticket fraud were instigated through Facebook and 6% on Twitter.
Almost a quarter (22%) of reported incidents took place on Gumtree.
Get Safe Online chief executive Tony Neate said: "Criminals are clever and often use pre-existing websites or fan forums to help them appear legitimate, or in fact mimic genuine websites to help them dupe their victims into handing over money.
"Take your time before making a payment and try to do as much research as you can to ensure that the provider or person you are buying from is exactly who they say they are.
"These criminals will jump at any chance to exploit innocent people, but it's worth remembering that their scams don't work without people handing over money."
City of London Police Commander Chris Greany said: "The newly released figures show that fraudsters will particularly target those who are spending large amounts of money on flight tickets or tickets for holiday packages.
"The fact that people in their 20s are most likely to fall victim to ticket fraud is concerning as this is the age group who are known to be most 'cyber-savvy'.
"If this group is falling victim it suggests that the fraudulent tickets sellers are very convincing and have the ability to exploit just about every type of internet user.
"We ask that people only buy tickets from official sites and when buying resold tickets ensure that they are buying from vendors who have been approved by the event organiser.
"If you do fall victim to a ticket fraud please report it to Action Fraud so we can identify how this criminality is being committed and shut it down."