Diamond Shortbreak Holidays – which includes The River Cruise Line and Diamond Rail Holidays – sold city breaks, river cruises, walking holidays and coach and rail trips.
It ceased trading earlier this month due to financial difficulties.
An estimated 7,000 bookings affecting as many as 16,000 people are understood to have been cancelled by the Market Harborough business.
It is believed to have employed 52 staff in the UK and another 26 overseas.
All customers who have booked packages will get a full refund, and travel organisations such as ABTA and the Civil Aviation Authority have began helping customers reclaim any money they are owed.