STA Travel has become the latest travel firm to fall victim to the Covid-19 pandemic.
The company, which grew out of a student travel business and specialised in trips for young people, including gap years and volunteer projects, has ceased trading.
STA Travel has more than 50 shops in the UK.
The firm said customers with bookings would "receive further communication in the coming days".
"We are sorry for the inconvenience and the limited information available to you at this time," a statement on its website said.
About 500 UK jobs are thought to be at risk as a result of the firm's failure.
The firm's parent company, based in Switzerland, said the pandemic had "brought the travel industry to a standstill".
A spokesperson for the Association of British Travel Agents (ABTA) said the news would "send a shockwave through the industry, bringing to life the very real pressures that travel is under at the moment".
"STA Travel will be a name that is familiar to most people who will have used them to travel or been aware of their name on the High Street, and this distressing news will sadly affect the livelihoods of hundreds of employees," the spokesperson said.
ABTA says the majority of flights and holidays sold by STA would be protected by the Atol scheme, an insurance scheme which protects holiday bookings. It directed customers to its website for further advice.
Amelia should have taken her month-long holiday to Bali and Borneo back in April but, when the pandemic arrived, it was postponed until September.
STA Travel told her and her boyfriend last week their trip would no longer go ahead at all. Now the 22-year-old from Walsall just wants to get her money back as quickly as possible.
"The STA agent said they would offer us credit notes but they would be split up between the different companies that STA booked all our travel and accommodation through.
"There is no way we would be able to spend all of the credit notes on the same trip if we did it ourselves.
"It's really, really disappointing - we just hope we can get our money back quickly but I'm not sure we will."
The Civil Aviation Authority said it was aware of "a number of consumers whose bookings have been cancelled by STA Travel Ltd as a result of government advice or flight cancellations".
Customers whose bookings were protected by Atol would be able to submit a claim through their online portal, the CAA said.
STA Travel, which originally stood for Student Travel Australia, but was later rebranded Student Travel Association, was founded in 1971, and specialises in long-haul, adventure and gap year travel.
The firm said: "Over recent months, the company took decisive measures to secure the business beyond Covid-19.
"However, sales have not picked up as anticipated, due to consumer uncertainties, further restrictions and renewed lock-down measures, which are expected to largely continue into 2021."
Simon Calder, travel editor of the Independent, said coronavirus had particularly hit long-haul specialists like STA, which arranged tailor-made trips.
He said a combination of High Street rents, a lack of income and demands for refunds was made worse when Australian airline Qantas announced it would not be running intercontinental flights in or out of Australia until the second half of 2021.
"Clearly the parent company… had to look at the future and just decided that there was no chance of business coming back at anything like the necessary amount before next year," Mr Calder said.
He added that "other casualties" were inevitable - particularly with countries being suddenly added to the UK's quarantine list.
"That's generated so much uncertainty that people simply aren't flying," he said.
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