Travel trade organisation Abta said more than 64,000 bookings have been cancelled and more than 2,500 jobs lost.
It added that the vast majority were coach package holidays which are financially protected, so customers with these bookings will receive a full refund.
Abta said it believed the Covid-19 pandemic was the main reason for SLG’s financial difficulties, as it struggled to provide thousands of refunds for cancelled trips while new bookings had ground to a halt.
The group’s operations also include brands such as UK Breakaways, the Bay Hotels, Country Living Hotels, Coast & Country Hotels, and Walla Arnold Travel. They have all ceased trading with immediate effect and the hotels will not reopen.
John de Vial, Abta’s director of membership and financial services, said: “The Specialist Leisure Group included two of the UK’s best known coach holiday brands, Shearings and National Holidays, two much loved holiday companies who for many years have provided holidays both at home and overseas to a very loyal group of customers.
“Today is a very sad day for these customers and the thousands of staff who will have lost their jobs.
“The fact that two such well-known brands with a loyal customer base have had to call in administrators is a stark indication of the pressure that the holiday industry is under as a result of the coronavirus pandemic.
“Abta has repeatedly highlighted to the government the urgency of the situation and the need to set out a coordinated strategy with clearer communication if it wants to help avoid significant job losses and support companies to weather the storm.”
Customers who booked a package holiday that included travel by coach or rail with overnight accommodation will have the payment protected by the Bonded Coach Holidays scheme.
Andrew McConnell, spokesman for Atol, which protects packages including flights, said: “This is a particularly sad day for customers and employees of Shearings Holidays Ltd, a long-standing business and well-known UK travel company.
“The company specialised in coach packages and other types of holiday bookings, however, there are a small number of consumers with flight-inclusive packages, which will be Atol protected.
“For these bookings, we will be contacting consumers directly or via their agent to provide guidance and support.”
Additional reporting by PA