Mental health charity Combat Stress has 662 Afghanistan veterans in its care and has been treating a rising number of soldiers who fought there.
Commodore Andrew Cameron, the charity's chief executive, said: "We cannot allow the ex-service men and women who suffer from the invisible injuries of war to go unnoticed and untreated.
"This is an unnecessary drain on society and our veterans and families deserve better."
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His warning came as officials projected 100,000 to 240,000 deaths in the US due to the coronavirus.
Three other people are confirmed to have died on the Zaandam, which has more than 200 Britons aboard.
NHS staff are frustrated as the Government fails to meet its target of 10,000 tests a day.