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".
The letter proposes four key actions, including the reopening of international travel for fully vaccinated travellers without the need to quarantine and the removal of all "unnecessary" testing requirements for countries on the green list.
It also said those travelling from amber list countries should only be required to take one Covid-19 test and the government should set a date to reopen overseas travel on Thursday (24 June) ahead of the first formal review of the traffic light system on 28 June.
Virginia Messina, WTTC senior vice-president, said: "Stalling the resumption of international travel until August could cost the country dearly.
"We simply can’t afford any further delay - we are running out of time and money, with many businesses facing going bust if international travel doesn’t resume in July."
