In a statement, XL, which trades under a number of names, said: "The companies entered into administration having suffered as a result of volatile fuel prices, the economic downturn, and were unable to obtain further funding."
Kroll, a company specialising in insolvency and acting as administrators for XL, said it was working to see whether the group had any future.
The company had operated flights to more than 50 destinations, mainly in the
Mediterranean but also the United States and Africa.
The CAA guaranteed to bring back people already on holiday who had been left without return flights.
Phil Wyatt, XL's chairman, said he believed his firm should not have been forced to go into administration and admitted it would be a "huge challenge" to bring home the estimated 67,000 people stranded outside Britain.
He said XL aircraft were sitting idle on runways at British airports, ready to repatriate holidaymakers, but that they were not allowed to take off.
"Unfortunately it's been deemed by the authorities - the government, whoever - that that's impossible. I personally don't believe that's the case," Wyatt told a news conference on Friday.
The chairman said the company had made "every effort" to find new funding for the business and he was "totally devastated".
Most customers would be offered repatriation flights or their money back if they had an advance booking, he said.
The CAA said 200,000 customers had advance bookings with XL.
Customers queuing at XL's main departure point for package holidays, London's Gatwick airport, faced a struggle to buy tickets for alternative flights to salvage their holidays, although many just went home.
Steve Simpson, 46, from London, was meant to be flying to Florida with his family.
He said: "We booked everything through XL Leisure Group - our flights, hotel, even our tickets to Disney World. We're absolutely gutted."
Ruth Kelly, Britain's transport secretary, said everything possible was being done to help get the holidaymakers home.
She said: "I have been holding discussions with my officials about this situation and both they, and the CAA, have been monitoring developments and putting rescue plans into action."
Britain's foreign office said embassies were providing "advice and information" to those affected.
XL, which carried 2.3 million passengers last year, has 1,700 employees worldwide.