Tickets for the football World Cup have been illicitly advertised for almost 40 times face value on secondary seller websites, according to a leading consumer rights group.
Fans heading to Russia risk being denied entry to matches if they show up with passes bought through a third party, Fifa has warned, saying it retains exclusive selling rights.
Tickets were on sale in March for as much as £5,618 each across five separate sites including StubHub and Ticombo, according to Which?.
A pair of premium category one tickets for the England v Tunisia match on June 18 were found listed for between £480 and £11,237 despite being advertised for £296 on Fifa's website, Which? said.
Alex Neill, managing director of home products and services, said: "Football fans need to be aware that if they buy a World Cup ticket from an unofficial source, they risk paying inflated prices and potentially not getting into the game at all.
"If you don't want to risk watching the World Cup from the sidelines, ?you should only buy from the official Fifa reselling website."
Fifa says it regards the "illicit sale and distribution of tickets as a very serious issue".
But, Ticombo said it was not violating any legislation by trading tickets.
A spokesman said: "If Fifa has a problem with fans wishing to sell their ticket to a third party, it has a problem not with Ticombo, but with the free market itself.
"Thus we do not acknowledge the legitimacy of the accusations put forward."
Fans who bought tickets from sites with consumer protection guarantees should go back to the companies, Which? added.
Fifa added there would be strict admission checks during the 2018 World Cup Russia and that a Fan ID document is needed for
A spokesman said: "A number of unauthorised online ticket sales, offered via websites and on social media originating from various countries, have been stopped during the past months.
"Furthermore, we have taken concrete legal action against a number of platforms... while encouraging fans not to purchase tickets from unauthorised sources."
StubHub said: "StubHub fully complies with applicable laws - therefore, we do not allow the resale of World Cup tickets on Stubhub.co.uk.
"Unfortunately, World Cup tickets were, due to a technical error, viewable from (but not purchasable on) our UK site for a limited period but this was promptly fixed. There are currently no tickets available for the World Cup on our UK site."
MP Julian Knight, a member of the Digital, Culture, Media and Sport Committee, said: "Ticket touting, such as what's been happening with these World Cup tickets, is a huge problem, making matches and events more inaccessible to fans.
"Someone needs to answer on who is getting their hands on these tickets to sell on in the first place and how.
"That's why beginning in the summer the Digital, Culture, Media and Sport Committee will continue our investigation into secondary ticket selling as part of our inquiry into live music and events."