“Yet we found ourselves in an army camp and we are surrounded by fences and security guards.”
The undated letter says that at least 120 cases of coronavirus have been identified at the barracks with more test results pending.
That figure was put to the Home Office, who have so far declined to comment.
A petition by Freedom from Torture to empty the barracks in Kent and Wales and close them down has racked up more than 10,000 signatures in less than two days.
Kolbassia Haoussou, lead survivor advocate at the charity, said: “A major crisis is unfolding in these unsanitary and dangerous places. Many of the people trapped here have low immune systems and mental health issues linked to the abuse they have fled.
“The Government has the power to end this nightmare now. Empty the barracks, close the camps, save lives.”
Bella Sankey, director of charity Detention Action, echoed calls to shut the barracks “before they are engulfed by tragedy”.
Immigration minister Chris Philp said: “We take the welfare of those in our care extremely seriously and asylum seekers can contact the 24/7 helpline run by Migrant Help if they have any issues.”
Use of Napier Barracks to house asylum seekers was initially authorised for six months under emergency provisions but the Home Office has said it is considering extending its use beyond the current deadline in around March.