In a letter to UK prime minister Boris Johnson, the global tourism body predicted a total loss of £19.8 billion if a resumption of international travel is delayed until August.
It also warned 218,000 more jobs in the sector are at "serious risk" of being lost.
The WTTC, in its letter, urged the government to "immediately unlock the doors" to international travel to avoid inflicting "severe long-term damage on the country’s economy".
