Booking website LateRooms.com and York-based holiday agent Superbreak - both owned by the Malvern Group - folded on Thursday.
Between them, they had some 53,000 customers.
In a statement, said: "We regret to advise you that Malvern Group incorporating Superbreak Mini Holidays Limited and Late Rooms Limited, has filed a Notice of Intention to Appoint an Administrator on 1st August 2019 and have ceased trading with immediate effect.
"It is the company’s intention to appoint an administrator to deal with the affairs of the business tomorrow."
Both LateRooms and Superbreak were still trying to drum up business in the hours before the announcement.
LateRooms tweeted on Wednesday it was "never too late for a cheeky weekend up in Manchester".
In a statement, LateRooms said anyone who had used its site and was concerned about a booking should contact the hotel directly.
It said: "In relation to your forthcoming booking(s), LateRooms.com acted as an agent on behalf of your accommodation supplier therefore we anticipate that your reservation is secure and your payment will be taken by the accommodation supplier either in advance (for non-refundable reservations) or at the property.
"LATE ROOMS LIMITED HAVE NOT TAKEN ANY PAYMENT FOR YOUR BOOKING
"We recommend however that you contact your accommodation supplier directly prior to travelling to confirm. You can find their contact details on your booking confirmation.
"To amend or cancel your booking you will need to contact the accommodation supplier directly."
LateRooms was not a member of travel trade body, ABTA. A statement released by ABTA said: "If you booked through a travel agent, please contact your travel agent for further advice.
"If you booked directly with Late Rooms, customers are advised to contact the hotel as we understand that payments were made directly to the hotels and therefore bookings should go ahead as planned."
ABTA said Superbreak was a member of the travel body. It said: "If you have booked a holiday with Super Break, we regret that all bookings that have not departed may be cancelled.
"The vast majority of holidaymakers’ travel arrangements will be covered through one of a number of different types of financial protection.
"If you are currently on a Super Break holiday, you should be able to continue with your trip as planned.
"Whilst this news may be extremely upsetting for you, please be assured if your holiday was booked with Super Break, we are here to help you."