As many as a quarter of tickets to popular music, theatre and sporting events end up on secondary ticketing websites, according to an investigation.
Tickets appearing on sites such as Viagogo, GetMeIn!, Seatwave and StubHub as soon as any major event goes in sale is now "the norm", Which? found.
It found that 26% of tickets for comedian Jack Whitehall's upcoming Eventim Apollo show ended up on four secondary sites, as did 17% of tickets for Lady Gaga at the O2 Arena in London and 15% of tickets for the first night of the BBC Proms at the Royal Albert Hall.
Tickets for the first night of the BBC Proms with an original cost of £38 were found to have a mark-up of 279% on StubHub (£144) and 300% on GetMeIn! (£152).
Which? also found consumers are still not getting the ticketing information required by law when buying from secondary websites.
It also found that half of people (49%) who bought tickets on these sites thought they were official sellers.
Consumers told the watchdog their two biggest problems when buying tickets on these sites were paying more than face value (72%) and hidden fees (46%), while 10% said the seat or area was not as described.
The Competition and Markets Authority (CMA) is currently investigating suspected breaches of consumer protection law in the secondary ticketing market.
Alex Neill, Which? managing director of home products and services, said: "People are finding themselves having to buy tickets through secondary sites more and more, and yet many struggle to find the basic information required by law.
"There needs to be more transparency within the secondary ticketing industry and the competition authorities must take strong action against those who aren't playing by the rules."
A StubHub spokeswoman said: "StubHub provides an open and transparent marketplace where fans can sell tickets for those events which they can no longer attend.
"According to our own figures, only 1% of tickets compared to the total venue capacity are listed for sale on StubHub, a much smaller number than the amount of tickets which are typically not put on public sale.
"These figures are misleading because many sellers will list on multiple platforms to increase their chances of selling, a very common practice in e-commerce.
"Prices also tend to drop as the event draws closer.
"For example, Jack Whitehall tickets for this weekend were selling on StubHub for £19.50, £10 less than the original cost."