New signatories include Sunvil, Cosmos, Oliver’s Travels, Simpson Travel and i-escape.
The campaign urges the government to set a date for reopening travel to restore consumer confidence that it is safe to book.
Save Our Summer is also calling for quarantine measures to be lifted by 1 May and replaced with a testing programme.
Campaign founder Paul Charles said: “We remain in discussions with other companies interested in joining and would urge everyone to sign up to SOS via the website.”
